feat: support injected languages

[alexpw]

Implements feature requested by #38

[alexpw]

^ The spec/filetypes/sql_spec.lua:125 succeeds and fails for me randomly, like a coin flip, locally. 🤯

[CKolkey]

Yeah, Ive had the same issue. Not sure whats up with it, and I don't think its related to this PR. Regardless, thanks for taking this on! I think it looks fine, so Ill try to merge it later today :)

[alexpw]

Happy to help!

Notes:

• lang This went with lang, because nodes are lang-based and now we have it available. I punted on fully moving to lang, because that would mean renaming the filetypes dir, which is a breaking change to people's custom setup()s (unless it uses a symlink and/or some method of deprecation).

I noticed a method, vim.treesitter.language.get_filetypes(lang), but it didn't make sense to me to use it, since filetypes are an alias for a lang and we already have it.

• get_node() As you'll see, this had to become public so it could be overridden in the spec_helper. I tried alternatives, to avoid this, but it meant reimplementing most of init.lua in the spec_helper, and this was by far the cleanest/simplest compromise.

[CKolkey]

Yea, the lang change makes perfect sense. And, agreed, there's no need to break people's setup for a semantic change like this :)

Didn't notice the change for get_node(). How about name it M._get_node() and add a ---@private comment?

[CKolkey]

Great work :)