feat: Generate table of contents code action

[keynmol]

Very basic demonstration of how a "Create TOC" can be implemented.
Note that I don't know F# at all :D

TODO:

1. Learn F#

2. Detect an existing TOC and update it when the code action is invoked (can even put diagnostics on the exact locations saying it's out of date)

   This is actually non-trivial if done in a non-invasive manner - only heuristic that can possibly exist is "first block of nothing but links in the document" and it can be easily broken. Can also match those links to a set of headings for some overlap number.

   

[artempyanykh]

Detect an existing TOC and update it when the code action is invoked (can even put diagnostics on the exact locations saying it's out of date)

This is actually non-trivial if done in a non-invasive manner - only heuristic that can possibly exist is "first block of nothing but links in the document" and it can be easily broken. Can also match those links to a set of headings for some overlap number.

@keynmol what do you think about generating toc with a heading ## Table of Contents (or with a slug of the heading being table-of-contents and then consider this heading and everything under it as a toc?

[artempyanykh]

Cool stuff!

[keynmol]

New version, capable of updating the existing TOC

[keynmol]

One issue with this is that currently Marksman's parser detects YAML frontmatter stuff like:

---
title: Hello
---

As a heading, and adds it to the TOC. I believe this is parser's deficiency, as it interprets the yaml block (which arguably isn't a part of the markdown spec) as one of the syntaxes for headings:

What makes matters worse is that --- is a horizontal line (https://spec.commonmark.org/0.30/#example-43), so you can't even technically parse the entire yaml frontmatter away :)

Markdown is hard

Only heuristic is really that the first line of the file is --- and there's a closing one as well, and the user somehow indicated that they want yaml frontmatter detection enabled

[artempyanykh]

One issue with this is that currently Marksman's parser detects YAML frontmatter stuff like:

@keynmol I think markdig (the lib we use) has some support for yaml front matter -- you may want to look into mark dig pipeline in Parser.fs, perhaps there's some extension that needs to be activated to fix front matter handling.

UPDATE: we can do it in a separate PR if you want. I'd rather merge this one and improve separately, so that GH stop considering you a "First time contributor" and making me approve running CI on your PRs 😄

[keynmol]

Ok, game plan:

1. Address the nits
2. Change the end marker to be another comment (helps with counting these damn newlines as well)
3. Merge
4. Enable yaml frontmatter and add tests in a separate PR

[artempyanykh]

Awesome! 🎉 Thanks for the contribution ❤️