Support for LSP completion requests

[marvinborner]

This PR is part of effekt-lang/effekt#384 and adds LSP completion request handlers for kiama.

I wonder if there's a more elegant (or already implemented?) solution for optWord? This is relevant for filtering the suggested symbols (e.g. by prefix). Just for testing I split by whitespace, but this should ultimately also consider dots, semicolons, etc.

[jiribenes]

What's the status of this PR? Can we merge this part and get to effekt-lang/effekt#384 later or is there something blocking this? :)

[marvinborner]

This PR depends on effekt-lang/effekt#384 's getCompletion, so merging only this wouldn't do anything.

The PRs suffer from the same problems as effekt-lang/effekt#398. Ideally, completion should only suggest symbols in scope. We don't currently have an efficient way of doing this.

Also, active typing typically introduces syntax errors, so caching the last working phase/stage might also be needed.

[jiribenes]

This PR depends on effekt-lang/effekt#384 's getCompletion, so merging only this wouldn't do anything.

I realise that, but my question is: will there ever be any more code for this on the Kiama side?
If we don't foresee more code here, then I'm fine with merging this PR...

[marvinborner]

Ah yes, I'm not sure. We may want a different API once Effekt's side is improved. It's probably fine though.