Support workspace symbols request

[Andriamanitra]

I think this would be a nice feature to have. The way I envision it working is

1. User enters a command symbols xyz
2. µlsp sends a workspace symbols request to the language server
3. when the language server responds with a list of symbols, open a "menu" (similar to what goto-definition has, where you can press enter to jump to the file the symbol is in)

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#workspace_symbol

[Andriamanitra]

The specification for workspace/symbol is rather similar to textDocument/documentSymbol (implemented in ef756ea) so some of the code can likely be copied and/or reused for this.