How do get Markdown-it plugins like https://github.com/lostandfound/markdown-it-ruby inside Discourse

markdown-it-plugin.user.js

// ==UserScript==
// @name         markdown-it-plugin for WaniKani Community
// @namespace    https://community.wanikani.com
// @version      0.1
// @description  markdown-it-ruby for WaniKani Community. Nothing more than necessary
// @author       polv
// @match        https://community.wanikani.com/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=wanikani.com
// @grant        none
// ==/UserScript==

;(function () {
  'use strict'

  const plugin = (function () {
    //...

    function ruby_plugin(md) {
      md.inline.ruler.before('text', 'ddmd_ruby', ddmd_ruby)
    }

    return ruby_plugin
  })()

  const markdownIt = require('pretty-text/engines/discourse-markdown-it')
  const oldCook = markdownIt.cook

  markdownIt.cook = function (raw, opts) {
    opts.engine.use(plugin)
    return oldCook.bind(this)(raw, opts)
  }
})()

markdown-it-ruby.user.js

// ==UserScript==
// @name         markdown-it-ruby for WaniKani Community
// @namespace    https://community.wanikani.com
// @version      0.1
// @description  markdown-it-ruby for WaniKani Community. Nothing more than necessary
// @author       polv
// @match        https://community.wanikani.com/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=wanikani.com
// @grant        none
// ==/UserScript==

;(function () {
  'use strict'

  const plugin = (function () {
    // Copied from https://github.com/lostandfound/markdown-it-ruby

    function ddmd_ruby(state, silent) {
      var token,
        tokens,
        max = state.posMax,
        start = state.pos,
        devPos,
        closePos,
        baseText,
        rubyText,
        baseArray,
        rubyArray

      if (silent) {
        return false
      }
      if (state.src.charCodeAt(start) !== 0x7b /* { */) {
        return false
      }
      if (start + 4 >= max) {
        return false
      }

      state.pos = start + 1

      while (state.pos < max) {
        if (devPos) {
          if (
            state.src.charCodeAt(state.pos) === 0x7d /* } */ &&
            state.src.charCodeAt(state.pos - 1) !== 0x5c /* \ */
          ) {
            closePos = state.pos
            break
          }
        } else if (
          state.src.charCodeAt(state.pos) === 0x7c /* | */ &&
          state.src.charCodeAt(state.pos - 1) !== 0x5c /* \ */
        ) {
          devPos = state.pos
        }

        state.pos++
      }

      if (!closePos || start + 1 === state.pos) {
        state.pos = start
        return false
      }

      state.posMax = state.pos
      state.pos = start + 1

      token = state.push('ruby_open', 'ruby', 1)
      token.markup = '{'

      baseText = state.src.slice(start + 1, devPos)
      rubyText = state.src.slice(devPos + 1, closePos)

      baseArray = baseText.split('')
      rubyArray = rubyText.split('|')

      if (baseArray.length === rubyArray.length) {
        baseArray.forEach(function (content, idx) {
          state.md.inline.parse(content, state.md, state.env, (tokens = []))

          tokens.forEach(function (t) {
            state.tokens.push(t)
          })

          token = state.push('rt_open', 'rt', 1)

          state.md.inline.parse(
            rubyArray[idx],
            state.md,
            state.env,
            (tokens = [])
          )

          tokens.forEach(function (t) {
            state.tokens.push(t)
          })

          token = state.push('rt_close', 'rt', -1)
        })
      } else {
        state.md.inline.parse(baseText, state.md, state.env, (tokens = []))

        tokens.forEach(function (t) {
          state.tokens.push(t)
        })

        token = state.push('rt_open', 'rt', 1)

        state.md.inline.parse(rubyText, state.md, state.env, (tokens = []))

        tokens.forEach(function (t) {
          state.tokens.push(t)
        })

        token = state.push('rt_close', 'rt', -1)
      }

      token = state.push('ruby_close', 'ruby', -1)
      token.markup = '}'

      state.pos = state.posMax + 1
      state.posMax = max

      return true
    }

    function ruby_plugin(md) {
      md.inline.ruler.before('text', 'ddmd_ruby', ddmd_ruby)
    }

    return ruby_plugin
  })()

  const markdownIt = require('pretty-text/engines/discourse-markdown-it')
  const oldCook = markdownIt.cook

  markdownIt.cook = function (raw, opts) {
    opts.engine.use(plugin)
    return oldCook.bind(this)(raw, opts)
  }
})()

overlay.user.js

// ==UserScript==
// @name         Editor overlay for WaniKani Community
// @namespace    https://community.wanikani.com
// @version      0.1
// @description  Editor overlay for WaniKani Community.
// @author       polv
// @match        https://community.wanikani.com/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=wanikani.com
// @grant        none
// ==/UserScript==

;(function () {
  'use strict'

  /**
   *
   * @param {string} raw
   * @returns
   */
  function fromVanilla(raw) {
    return raw
  }

  /**
   *
   * @param {string} raw
   * @returns
   */
  function toVanilla(raw) {
    return raw
  }

  /** @type {HTMLTextAreaElement} */
  let elTextArea = null
  const getTextArea = () => document.querySelector('textarea.d-editor-input')

  function initTextArea() {
    if (!(elTextArea = getTextArea())) {
      const obs = new MutationObserver(() => {
        if ((elTextArea = getTextArea())) {
          obs.disconnect()
          onNewTextArea()
        }
      })

      obs.observe(document.body, {
        childList: true,
        subtree: true
      })
    } else {
      onNewTextArea()
    }
  }

  function onNewTextArea() {
    elTextArea.value = fromVanilla(elTextArea.value)

    const endObs = new MutationObserver(() => {
      if (!getTextArea()) {
        elTextArea = null
        endObs.disconnect()
        onCloseTextArea()
      }
    })
    endObs.observe(document.body, {
      childList: true,
      subtree: true
    })
  }

  function onCloseTextArea() {
    initTextArea()
  }

  let alreadyInjected = false
  function injectIntoDiscourse() {
    if (alreadyInjected) return
    alreadyInjected = true
    // greasemonkey workaround: unsafeWindow + exportFunction
    let w = typeof unsafeWindow === 'undefined' ? window : unsafeWindow
    let e = typeof exportFunction === 'undefined' ? (o) => o : exportFunction
    injectCustomCook(w, e)
    injectCustomSave(w, e)

    let oldCook = w.require('pretty-text/engines/discourse-markdown-it').cook
    w.require('pretty-text/engines/discourse-markdown-it').cook = e(
      (raw, opts) => oldCook(toVanilla(raw), opts),
      w
    )

    let oldSave = w.require('discourse/controllers/composer').default.prototype
      .save
    w.require('discourse/controllers/composer').default.prototype.save = e(
      function (t) {
        tText.value = toVanilla(tText.value)
        tText.dispatchEvent(
          new Event('change', { bubbles: true, cancelable: true })
        )
        oldSave.call(this, t)
      },
      w
    )
  }

  injectIntoDiscourse()
  initTextArea()
})()