Add nim language support.

[theherk]

Fixes #3117.

I had some spare time, so I just grabbed the top task in the queue as a first. I'm a bit confused though because the language server does support completions, but that check doesn't get added. Further, even though I've linked the runtime in the branch and fetched and built grammar, that isn't checked either. In --health I get:

nim                  ✔ nimlangserver      None                 ✘                    ✘                    ✘

Not entirely sure why. Any ideas?

[the-mikedavis]

In order to perform syntax highlighting, indents and textobjects we also need queries https://docs.helix-editor.com/guides/adding_languages.html#queries

The tree-sitter-nim repo doesn't have example queries so you'll need to write them from scratch, or maybe nvim-treesitter has some to go off of.

See also #2633

I'm a bit confused though because the language server does support completions, but that check doesn't get added.

Those healthcheck checkmarks don't check for completion support: https://github.com/helix-editor/helix/wiki/Healthcheck#language-configuration-section

[theherk]

I understand more fully now. I'll remove and reopen when queries are complete.