feat: workspace symbols warmup (Phase 2)

[AlexCannonball]

Improvements related to Phase 2 from #130

[AlexCannonball]

At the moment still a very early attempt to implement eager warmup + lazy query evaluation from #130 (comment)

TODO list from the top of my head:

• Add progress to the warmup function
• Get rid of the temporary debug ProtoLanguageServer::cache field
• Implement the CacheEntry state change methods
• spawn_blocking to get metamodel for document(s)
• Event listeners concept from Proposal: High-Performance Architecture Refactor (Zero-FFI Queries, Pre-Computed Metadata) #130 (comment)
• FS watchers to handle things like git checkout
• handle adding and removing Workspace Folders, with nested folders optimizations

[AlexCannonball]

Quick update on Phase 2 refactoring:

I'm currently pushing all heavy synchronous workloads (Tree-sitter processing, WalkDir, protoc, and clang-format) completely out of the main current_thread executor.

As expected, introducing async and graceful cancellation has deeply "infected" the entire codebase.

Right now, the IDE is reporting 100+ cascading type mismatch and lifetime errors.

I am currently grinding through these compiler errors layer by layer to align all the type boundaries and will continue pushing updates to this PR as soon as the core compilation passes. Still on it!