Highlighting inside of recipes

[antler5]

This might depend on the grammar we're using, but it would be nice to offer even some rudimentary parsing for sh, bash, and Guile. Idk much about tree-sitter, but the :language key seems to indicate some existing mechanisms for this?

[antler5]

I tried to see what the parse tree looks like for an invocation of $(guile [sexp]), but run into upstream issue #8. It's reportedly not an issue on the "master" branch (as opposed to "main"), but I get a compine-time error about RECIPEPREFIX_VAR not being a constant expression when used as a case condition-- I can see that it is a constant variable, but know that's not necessarily the same thing as a constant expression. Unfortunately I don't have more time for that right now.

Leaving this issue open because I may still be able to implement sub-parsing for SH.