Editor completions for extensions to builtin types

[fhammerschmidt]

Extend rescript.json with an editorconfig for autocompletion, e.g.

"editor": {
  "autocompleteConfig": {
    "array": ["ArrayUtils"]
  }
}

So that the methods of that module get autocompleted the same way as the @editor.completeFrom types would.

[zth]

The example is for a builtin type, but I think we could allow it for other types that you can't access to add directives directly in your project as well. For example, say that you have fastify installed:

"editor": {
  "autocompleteConfig": {
    "Fastify.t": ["FastifyExt"]
  }
}

[fhammerschmidt]

Starting point: #7330