Improve Treesitter heading highlighting for Markdown

[SWiegandt]

I was a bit confused why #161 seemed to have disappared, but you mentioned having force pushed in a discussion thread, so I'm assuming it's because of that. Resubmitting the same change again, and hopefully it will stick this time. 😄

[mvllow]

Ugh, sorry about that. I appreciate you making the change again 💜

[attilarepka]

Hi, @mvllow
Could you please double-check if this MR works properly on latest(v0.9.0 on my setup) neovim?

For me, after :PackerSync (package updating) to latest rose-pine, it tries the fetch the following commit, however, this seems to be non-existent mainline main branch.

 ✗ Failed to update rose-pine/neovim
  Errors:
    fatal: Not possible to fast-forward, aborting.
    15aac8c

P.S.: Did not want to raise an issue until this gets verified by someone.

Thanks in advance.

[mvllow]

Sorry it may be caused by me force pushing at some point—#162 (comment)

[attilarepka]

No worries, it worked! Thanks a lot!