✨ Added coq support

[HiiGHoVuTi]

Only added the lsp support as I found no grammar.
I am open to suggestions, as this is my first time adding a language to Helix and I'm no coq expert.
I ran tests locally and updated the documentation.

[the-mikedavis]

Rather than adding a language entry for this let's recommend it in the language server configs wiki: https://github.com/helix-editor/helix/wiki/Language-Server-Configurations

We generally don't accept languages that are language-server configurations only and the "v" file-type will conflict with vlang and verilog

[edwintorok]

Based on this I added the following entry to my ~/.config/helix/languages.toml:

[language-server.coq-lsp]
command = "coq-lsp"

[[language]]
name = "coq"
scope = "source.v"
file-types = ["v"]
block-comment-tokens = { start = "(*", end = "*)" }
language-servers = [ "coq-lsp" ]
indent = { tab-width = 2, unit = "  " }

And this gets some basic support working (no syntax highlighting though!).

However looking at coq-lsp it supports quite a few extensions to the LSP protocol that are useful when working with proofs: https://github.com/ejgallego/coq-lsp/blob/main/etc/doc/PROTOCOL.md#extensions-to-the-lsp-specification
I think the extensions would have to be implemented by Helix (or as a Helix plugin if/when Helix supports them)

For syntax highlighting would semantic tokens (once implemented ejgallego/coq-lsp#854 (comment)) work, or would a tree-sitter grammar be needed?
I only see this grammar: https://github.com/ejgallego/coq-lsp/blob/main/editor/code/syntaxes/coq.json and I'm not sure if that can be converted into something that Helix could use.