feat: Add syntax highlighting for Emacs

[vanillajonathan]

Adds a major mode for the GNU Emacs text editor which provides basic syntax highlighting.

It is basic. It doesn't use treesitter, nor does it provide code folding, formatting, REPL or anything.

[max-sixty]

I thought that emacs supports tree-sitter now? Or not some versions / I'm mistaken?

[vanillajonathan]

Yes it does, modern versions of GNU Emacs (since version 29.1) now supports tree-sitter but the mode in this PR does not make use of the tree-sitter because I don't use Emacs and I don't know Emacs Lisp or Lisp so I wouldn't know how to do anything like that, hence I made just a very basic mode.

[max-sixty]

OK, no strong objection to merging, but I'm guessing folks will just use the tree-sitter grammar rather than this... I had thought that if there's a tree-sitter grammar, then users can add it (but I also don't use emacs)

[vanillajonathan]

I don't know how users would add it, maybe they could contribute to this script to make it use tree-sitter and other features such as code formatter, code folding, indention and REPL.