Rust analyzer support commands

[trevarj]

Rust-analyzer has these "extension" commands, which I do not believe are implemented yet with the built-in LSP.
Here is an example of one and also the list of them provided by coc:
https://rust-analyzer.github.io/manual.html#expand-macro-recursively
https://github.com/fannheyward/coc-rust-analyzer#commands

This can be closed if this is not the correct repo to implement these.