Support fluent fragments in yaml in the IDE with language injection

[samreid]

To help read and write fluent fragments in the IDE, it would be helpful to have syntax highlighting. I was able to get partial support via this approach:

Add this JSON schema file to the repo:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$id": "i18n-a11y.schema.json",
  "title": "i18n a11y Fluent injections",
  "allOf": [ { "$ref": "#/$defs/node" } ],
  "$defs": {
    "injectString": {
      "type": "string",
      "x-intellij-language-injection": {
        "language": "Fluent",
        "prefix": "tmp = "
      }
    },
    "a11yNode": {
      "anyOf": [
        { "$ref": "#/$defs/injectString" },
        {
          "type": "object",
          "additionalProperties": { "$ref": "#/$defs/a11yNode" }
        },
        {
          "type": "array",
          "items": { "$ref": "#/$defs/a11yNode" }
        }
      ]
    },
    "node": {
      "anyOf": [
        {
          "type": "object",
          "properties": {
            "a11y": { "$ref": "#/$defs/a11yNode" }
          },
          "additionalProperties": { "$ref": "#/$defs/node" }
        },
        {
          "type": "array",
          "items": { "$ref": "#/$defs/node" }
        },
        { "type": ["string","number","boolean","null"] }
      ]
    }
  }
}

Use the JSON schema associator to associate the yaml with this json schema. Enable language injection plugin (IntelliLang, I think), Enable Fluent plugin.

Then it looks like this:

It correctly matches braces and uses fluent syntax on those fragments.

See how it matches braces:

However, it lists our custom syntax { a11y.gameScreen.myText } as a parse error since that is not part of fluent, and it does not handle multilines (a string constant that spans 2 lines with yaml |- syntax), and it reformats oddly. So I don't know how to move forward with it at this time, just wanted to leave a note in case we can overcome these flaws in the future.

[samreid]

The proposal in #1645 moves the majority of our fluent selectors to yaml syntax, so the problem in this issue is much less severe.