feat: add semantic analyzer

[vhyrro]

This PR aims to implement a fully fledged semantic analyzer into Neorg.

With such awesome tech we could do much more than currently possible with simple treesitter queries, i.e.:

• Create LSP-like features (enumerate dead links, show link previews etc.)
• Understand correlations between headings
• Zettelkasten (by using the above feature to track backlinks across files)

I'm very excited to take on this challenge, but as you can imagine it's proving itself very difficult. Might take a while, might not. We'll see :p