Provide support for nVim's native completion

[diktomat]

Issues

• [X] I have checked existing issues and there are no existing ones with the same request.

Feature description

The completion module currently only works with compe (and cmp?). Users accustomed to nVim's native ^X^foo can use file, keyword, spelling … completion of course, but it would be great if Neorg could ship omnifunc and tags completion for use without any completion plugin, too.

Help

No

Implementation help

Sorry, but my Lua skills are barely enough for Neovim config..