Add support for nixd to ALE (vim/neovim)

[lloeki]

Is your feature request related to a problem? Please describe.

nixd describes coc.nvim as a possibility for Vim/Neovim integration

But coc.vim may be undesirable in one's setup (e.g it spawns a whole Node.js); comparatively, ALE is implemented in pure vimscript and relies only on first-party vim/neovim features.

Describe the solution you'd like

nixd should work OOTB when using ALE and nixd is installed. Usage should be documented here.

I can try to implement the ALE part, but if someone beats me to it then great!

Describe alternatives you've considered

Using rnix_lsp [1] [2] but this is what is said of it in its README:

This is beta-level quality at best - I didn't expect maintaining a language server when writing rnix, the goal was that others would flock around the parser and write a bunch of editor tooling :)

Oh, and the repo is archived, so...

Additional context

This issue is for cross-tracking/awareness, implementation should hopefully require no change to nixd.

Issue at ALE: dense-analysis/ale#4839