Complete payload of @module attribute

[zth]

Adds basic completion for the string payload of the @module() attribute. A (pretty weird) example:

This is a basic version that will complete for:

• Package names in dependencies + devDependencies of package.json, if it can find it
• Local .js and .mjs files next to the source ReScript file

Ideas building on this:

• Resolve relative paths (completing ../ gives that folder contents, etc)
• Resolve .d.ts files relative to packages. Completing next/ could give next/link, etc

One could maybe even go further and resolve @scope in combination with @module, and the external assignment, like:

@module("next/link") external make: ... = "<com>"

And via that look up what exports there actually are in the next/link package.

One can dream at least. This is a first small step.