Implement LSP autocomplete

[marvinborner]

This PR closes #367. Together with effekt-lang/kiama#1 and effekt-lang/effekt-vscode#16, this should result in first steps towards more intelligent LSP intelligence.

Main things still missing:

• Better symbol selection; for now all suggested completion items are selected by prefix of the currently typed word
• Only suggest symbols in AST within the scope of the current position; this is especially hard since the tree often has syntax errors while completing; for now all symbols in the AST get suggested
• Be much more smart about completion suggestions, e.g. dot notation should only suggest relevant sub items, else should only be shown if within if, effect handlers could be autocompleted (LSP can write entire snippets) etc. This is probably too much for this initial PR.

I very much welcome criticism and suggestions for elegant solutions. I'm still unfamiliar with Scala and the code base.