Lumbar mixin terminology update

library.json

{
  "name": "something-unique",

  "mixins": {
    "base": {
      "scripts": [
        "global.js",
        "base.js"
      ]
    },
    "other-shit": {
      "scripts": []
    }
  },

  "global-setting": "value"
}

lumbar.json

{
  "modules": {
    "home": {
      "mixins": ["base"]
    }
  },

  "libraries": [
    "libraries/library.json"
  ]
}