Searching for workspace symbols can be very slow in large projects. We should add some form of caching to workspace symbols to traversing every single file on every request.

I don't think it's necessary to build a full search index. Using only salsa for caching should probably be fine for an initial version:

• Make symbols_for_file a salsa query that doesn't take any option and returns all symbols of that file. We could consider making it two queries: One that returns hierarchical and another that returns non-hierarchical symbol information. The most important part is that the query isn't parametrized by the query string. Instead, filtering should happen after collecting all symbols. This gives us per-file caching, which should make subsequent lookups very cheap
• Iterate over all files in workspace_symbols and apply the filtering based on the user input