Add option to use treesit for markdown-mode

[CeleritasCelery]

As you well know, Emacs 29 is now shipping with tree-sitter support. There is a tree-sitter grammar for markdown that implements the CommonMark Spec. It could potentially be used as backend for markdown-mode. It is already used by neovim and helix.

[milanglacier]

As also a neovim user, I would say the treesitter parser of markdown is still far from maturity, at least for the syntax highlight the treesitter neovim is worse than the pandoc-syntax.

While there are no obstacles for adding treesit support for markdown-mode, but I would say the cost is higher than the pay for the current markdown parser, as I can foresee that the current one would still provide better font-locking.