Syntax highlight the JS in `%raw`

[cristianoc]

E.g.

let canUseCanvas: unit => bool = %raw(`
  function canUseCanvas() {
    return !!document.createElement('canvas').getContext;
  }
`)

could syntax highlight the JS code.

This issue could be a combination of finding an alternative surface syntax, e.g.

js`() => 3`

and adapt the editor extensions's grammar to recognise this syntax and inject a JS syntax highlighting grammar.

[cristianoc]

CC @fhammerschmidt

[cristianoc]

OK write it down concisely now so I can paste it in a GitHub issue in markdown format.

To add custom syntax highlighting for embedded JavaScript code within your custom language in a VSCode extension, follow these steps:

1. Update the tmLanguage.json file in your extension's syntaxes folder with the custom rule:

   {
     "name": "source.embedded.javascript",
     "begin": "\\bjs`",
     "end": "`",
     "beginCaptures": {
       "0": {
         "name": "punctuation.definition.string.begin.embedded-js"
       }
     },
     "endCaptures": {
       "0": {
         "name": "punctuation.definition.string.end.embedded-js"
       }
     },
     "patterns": [
       {
         "include": "source.js"
       }
     ]
   }

   This rule tells VSCode to recognize the custom syntax pattern and apply the JavaScript syntax highlighting to the code enclosed within the backticks.

2. Test the changes in VSCode using the Extension Development Host to ensure the custom syntax highlighting works as expected and does not conflict with existing syntax rules.

3. Once the changes are working correctly, update the version number in your package.json file and publish your updated extension to the Visual Studio Code Marketplace.

Note: This assumes you have already created a language extension with its own grammar file, and that the built-in JavaScript syntax rules are available in your environment through the source.js scope.

[cristianoc]

[cristianoc]

Don here: rescript-lang/rescript-vscode#774