feat: Add specific markdown syntax highlighting

[bigpick]

Info

see: https://github.com/pineapplegiant/spaceduck/issues/48

Currently Shipped w/vim

After these updates:

---

For reference, the Atom syntax readme highlighted that we had decided on before:

[bigpick]

Other

So, this is kinda based off how Dracula does theirs: https://github.com/dracula/vim/blob/master/after/syntax/markdown.vim (as opposed to how the plugin currently is, which looks alot more like say, nord's)

Notice, they have quite a bit different structure; personally, I like splitting the languages out into their own file under after/syntax vs putting them in the colors file, as it makes maintaining the individual languages much more clearer, and is a bit easier to see what has been done specifically.

[jinh0]

Looks good! I apologize for my misunderstandings over your code, I will try to be more familiar with Vimscript 😅.

I'll merge this pull request after @pineapplegiant makes the executive decision to separate syntax into language-specific files.

I think once it's in the dev branch, we can work on improving the highlight function together.