feat(completion): blink.cmp support

[guspix]

From upstream:

Adds support for the blink.cmp completion plugin. The nvim_cmp sources were refactored to reduce code duplication. The source/provider injection to the Blink configuration was rather hacky, but it works with functions as well. I tested the functionalities with both plugins, I was able to reference existing notes, with both syntax, create a new note, and add tags, although I might have missed something.

Closes epwalsh#770