Extract SymbolIndex to project crate for Tree-sitter based symbol search

[portseif]

Summary

Extracts the symbol indexing functionality from Search Everywhere into a reusable symbol_index module in the project crate. This enables Tree-sitter based symbol search for languages that don't have LSP support.

Key changes:

• New symbol_index.rs module in crates/project/
• SymbolIndex struct with cached symbols per project
• Incremental updates via buffer and worktree subscriptions
• search() function for fuzzy matching symbols
• Indexes outline items from Tree-sitter (works without LSP)

This is a foundational piece that can be used by Search Everywhere and potentially other features that need project-wide symbol access.

Test plan

• Build passes (./script/clippy -p project)
• Manual testing with Search Everywhere using the new API

[maxbrunsfeld]

I am open to this change. My requirements:

• Biggest question - To merge this, it will need to already be exposed as an end-user-facing feature somehow. I think the "Search Everywhere" thing does not exist yet. @portseif How do you envision this working, given that we often have LSP-based symbol search? Would we only index the files that are not covered by an LSP? Or do we index all files, and show both sets of symbols, de-duped somehow?
• I'd like this to live in a separate crate, not project. I know a lot of stuff lives in project, but we want to move stuff out, as project is so central on the crate graph.
• This will need to handle files changing on disk (e.g. I run git pull, lots of symbols change), not just open buffers being edited.
• Efficiency of updates is going to be important. When the project is huge (e.g. chromium, linux), how do we efficiently update the index when one file changes? Currently, it seems like rebuild_all_symbols would be very slow.
• This will need good tests. The tests need to exercise files updating, editing buffers etc.

[maxbrunsfeld]

Left my changes in the comment above.

[maxbrunsfeld]

I'm going to close this out for now. Feel free to open a new PR if you want to keep working toward addressing the feedback. Thank you!