feat: add omnifunc completion source

[FerretDetective]

Closes #1103

[FerretDetective]

A few thoughts:

1. Should this source have any specific 'kind' for its completions?
2. Should this to be opt-in, or enabled by default?
3. Would it possibly make more sense to instead implement a general source for all complete-functions (completefunc, thesaurusfunc, and omnifunc)?

[milanglacier]

Should this to be opt-in, or enabled by default?

I think this should be opt-in. Omni-completion seems to be a synchronous interface? And neovim will automatically set lsp as omni-func by default when lsp is attched and omni-func is not set by the users. Probably don't want to have double-lsp.

[FerretDetective]

Omni-completion seems to be a synchronous interface?

The complete-functions does seem to be synchronous, though it does mention using complete_add() and complete_check() for "when searching for matches takes some time." I don't know how much that matters for this application.

And neovim will automatically set lsp as omni-func by default when lsp is attched and omni-func is not set by the users. Probably don't want to have double-lsp.

By default I have v:lua.vim.lsp.omnifunc disabled because:

1. Double-lsp
2. It just doesn't work when enabled. For some reason invoking it causes it to hijack control and opens the <C-x><C-o> popup. This happened when I tested cmp-omni as well.

Either way, I don't disagree that it may make more sense for this to be opt-in.

[Saghen]

Thank you! Opt-in makes sense since the API is synchronous

Would it possibly make more sense to instead implement a general source for all complete-functions (completefunc, thesaurusfunc, and omnifunc)?

I'm open to renaming this omni source to make it clear that it's for all complete funcs. I likely won't have time to work on this but feel free to PR if that interests you

[FerretDetective]

I think—at least for now—it might be best to leave it at omnifunc to avoid the complexity of having multiple complete-functions being used through a single blink source. Though, if it is requested I can have a look at submitting a PR for that.