project: Fix `SymbolKind` serialization over RPC (#62458)

# Objective

I noticed that in workspace symbol search, the function's symbol kind
has become `Trait`.


## Solution

Add `to_proto` and a macro to define the mapping instead of `as i32`.

## Testing

Updated the test.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

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Release Notes:

- Fixed `SymbolKind` mapping to LSP protocol values