feat: go-to-definition, fix bugs, update deps, remove unused files

[aldur]

I am opening this PR as draft since there is a big intersection with #634 and it probably does too many things at once. I have also cherry-picked a couple commits that have since been pushed to develop. I have rebased it on develop so that it should include all changes.

If interesting we can split it into multiple PRs; the intent is to kickstart the conversation here.
I am opening it against develop and leaving review comments inline to explain the rationale behind the changes.

For context, I have been using this project for a while, thank you for creating it!

This PR:

1. Implements LSP go-to-definition for accounts and commodities.
2. Fixes bugs related to references and renames, while making them re-use existing code (for instance, using the code to fetch references and then renaming them, instead of duplicating it).
3. Turns default log-level to INFO (this was making it hard for nvim to cope when editing a big Beancount file).
4. Adds some more testing and extracts the testing utils to a different file, so they can be re-used across tests / modules.
5. Updates to the latest tree-sitter version supported by the grammar, which introduced some light breaking changes.
6. Where possible, uses idiomatic Rust e.g. to create collections from iterators.
7. Unifies the naming of providers (some started with handle_, others did not).
8. Updates the README with the latest developments.
9. Removes a ton of files that, to the best of what I could see, are unused?

I have tested my changes against nvim. I might have obviously missed things, but have tried to add a few tests to increase confidence.

[trim21]

maybe we can close this now?

[trim21]

sorry, only "goto account" feature is merged already, other features are still not supported

[polarmutex]

This is a large draft PR that's been open since July 2025. It includes go-to-definition improvements, bug fixes, and dependency updates, but it's a DRAFT and predates significant recent work (including PR #824 which overlaps on go-to-definition).

Given the overlap with current development and the draft status, I'm marking this as stale. @aldur — if you'd like to continue this work, please rebase onto main and remove the draft status. Otherwise I'll close it in 2 weeks to keep the PR queue clean.

[aldur]

Thank you! Is there anything in particular you'd like to keep? I can split / prioritize based on what you have in mind for the project.