feat: auto complete links

[benlubas]

Adds auto completion for links to the completion module.

closes #752

screen_cap.mp4

Here's a list of the completions:

• {:|} workspace relative file paths
• {#|} all headings in the current file
• {*|} level 1 headings in current file (TODO status is removed)
• {**|} level 2 headings in current file (and so on)
• {^|} footnotes in the current file
• {:path:#} headings from the file at path
• {:path:*} level n headings from the file at path
• {:path:^} footnotes from the file at path. Will find ^ footnote and ^^ long footnote

All of these should work with spaces after the item (ie. {# |} will still complete correctly).

All of them should close the link correctly if it's not closed (ie. selecting a completion when the text looks like this: {:| will close the link with :}).

---

• Currently the completion source shows as text. I'm not sure how to change that. It would be nice to change them to like Keyword or Symbol or something so that it's more clear that these are valid headings/paths being suggested by neorg, not just text from the buffer.

• There might be more links that I'm missing, I want to make sure I've covered them all.

• this might be really slow in large workspaces. It's not a problem for me though, but admittedly I don't have a ton of notes.