Fix highlighting in string escapes for emacs tree-sitter-mode

[talyz]

Reason

tree-sitter-mode seems a bit picky about the order in which expressions are listed in highlights.scm and won't highlight expressions inside string escapes correctly without this change. Note: I haven't tried the change with any other editor.

Before and after

[adisbladis]

@cstrahan Since you're not super active in maintaining this repo would you mind if I merged this PR, and possibly others in the future too?

[adisbladis]

I plan to merge this in a few days unless I get feedback from @cstrahan to the contrary.

[cstrahan]

@adisbladis You have my blessing to merge whatever you wish going forward 👍. I've been oversubscribed for a little too long now, and getting hit with a second layoff (just a year after my first) doesn't help. Whenever things chill out in my life I'd love to be more active here and in other corners of the Nix ecosystem, but I dunno when that'll be...