Support for DocumentSymbols

[tadfisher]

Some servers support DocumentSymbols but not WorkspaceSymbols. Could we get a Consult command for that?

[mohkale]

Is there a need for something eglot specific here? eglot integrates with imenu through the DocumentSymbols call. So whenever you run imenu to jump to definition DocumentSymbols will be queried. Consult also has direct interface with imenu through consult-imenu so you can transitively resolve document symbols through consult-imenu.

The only missing piece is consult-imenu doesn't really do asynchronous the same way consult-ripgrep and friends so it can block on especially large files or slow servers.

I think we should just add a comment to the README to explain this, but I'm not sure dedicated support in this package is required.