How to add my own auto-completion source?

[milanglacier]

Describe your feature request

It seems that currently only lsp provides auto-completion source.
Buf similar to nvim-cmp, I would love to see we can provide more completion source, especially words in buffer. This is very useful when writing markdown file.

I would love to try to make contributions, but I am not sure where's the entry point of hacking auto completion module.

[the-mikedavis]

Custom completion sources have an issue here #1015 which is mostly scoped towards keywords. More advanced completion providers would probably be scoped as plugins #122