app.ts

import { createApp, configureCompat } from "vue"

configureCompat({ MODE: 3 })

const app = createApp({
  setup() {
    return {
      // stuff
    }
  }
})

vue-compat.d.ts

declare module "vue" {
  import { CompatVue } from "@vue/runtime-core"
  export const configureCompat: CompatVue["configureCompat"]

  // TODO: How do I make this not necessary
  export * from "@vue/runtime-dom"
  export { }
}