feat: LSP autocompletion for use statement

[asterite]

Description

Problem

Part of #1577

Summary

Implements autocompletion for use statements, so typing anything after use.

lsp-use-autocompletion.mov

Note: the gray hints are probably coming from copilot, I should have turned it off before recording 😅

Additional Context

To begin implementing autocompletion I thought starting with use would be the easiest thing, but also not having to look up the std (or your project) module hierarchy could be useful.

To implement this I had to allow parsing use foo::, that is, allowing trailing colons, because otherwise that wouldn't parse to a use statement and we wouldn't have any information to do the autocompletion.

There are still some cases that don't work. For example if you write use and explicitly ask for autocompletion, you don't get anything. The reason is that use gives a parse error so no AST is produced. We could tackle those in later PRs (I didn't want to continue making this PR bigger).

This PR also introduces a simple way to test autocompletions, so next autocompletion PRs should be much smaller.

Another thought: at first I was worried that caching def_maps in the LSP state would increase the memory usage by a lot, but in retrospective I think that's not a heavy data structure, or at least NodeInterner, which was already cached, is probably much larger in memory footprint. I tried this PR in a few projects and the memory usage was fine.

Documentation

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[asterite]

Sorry for the double review request, I noticed it wasn't working well in some cases so I fixed it before requesting a review.

[jfecher]

@asterite should this still be a draft?

[asterite]

@jfecher Nope, forgot to undo that too. Thanks! It's ready for review :-)

[jfecher]

👍