5516: Better LSP conformance r=matklad a=vsrs
At the moment rust-analyzer does not fully conform to the LSP. This PR fixes two LSP related issues:
1) rust-analyzer sends predefined server capabilities and does not take supplied client capabilities in mind.
2) rust-analyzer uses dynamic `textDocument/didSave` registration even if the client does not support it.
Co-authored-by: vsrs <vit@conrlab.com>