rust-analyzer: search and replace

[csmoe]

SSR can be invoked in coc.nvim, is it possible to do that with nvim-lsp? (it's fine to say no as SSR is a non-official feature of LSP)

cc https://rust-analyzer.github.io/manual.html#structural-search-and-replace

rust-analyzer ssr <RULE>

[mjlbach]

My general thought is not to add non-spec commands, but for servers in which having a command drastically improves usability (see texlab) it's fine. If you think ssr is a very important feature, then make a PR and I'll merge it.

[csmoe]

rust-lang/rust-analyzer#7874
rust-analyzer can apply ssr with lsp.code-action now 🚀