feat: ws relative file/header backlink pickers

[benlubas]

closes #49

This is the "backlink picker at home". It's a simple, incomplete, implementation that is "good enough".

screen_cap.mp4

It does:

• find workspace relative backlinks to the current file or header
• uses rg regex to find links
• uses lua patterns to find the current heading (searches up until it hits one)

It does not:

• handle relative path links
• use TS to find the current heading
• find links to footnotes

I, and others want a simple backlink functionality now while we wait for these features to be properly implemented in Neorg. So that's what this is, something to hold us over while we wait for the real thing.

[benlubas]

Oh, still have to add docs. will do that later tonight

[benlubas]

This is now ready to be merged (pending review)