Adding a new LSP server example

[alanwsmith]

The current LSP example is a good, minimal demonstration. It show how to respond to textDocument/definition requests.

I used it to make an example that includes capturing content from textDocument/didChange and using it to provide textDocument/completion and textDocument/formatting functionality.

My version is here:

https://github.com/alanwsmith/rust-grimoire/tree/main/rust_analyzer_lsp_example

It's more of a head start for getting up and running. I'd be happy to add it to the rust_analyzer examples if other folks would find it useful.