syntax formatting for m4

[ebblake]

I noticed that while you have syntax formatting for a number of languages, you are lacking it for m4. There are a few keywords (such as define, incr, substr, ...), comments (# to newline), as well as substitutions like $1, $2 which at least the emacs mode for m4 is able to colorize. m4's heavy use of nested `' strings is not worth colorizing. Adding color to be similar to emacs would make this code easier to read

[bokub]

Hi 👋
The syntax highlighting feature comes from CodeMirror, which doesn't support m4 unfortunately.

However, you can post a new issue on their repo, asking for a "m4 mode", you'd be the first one!