feat: Add (rudimentary) support for listings package

[atticus-sullivan]

Addresses #20

I'm open for suggestions (@walshyb maybe).

Without standardization (e.g. keyword group 1 is always for keywords, group 2 always for builtins etc) from the listings package, I see no way of providing a better highlighting than this PR implements so far.

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For me the one thing I want to investigate is to check how we can avoid this "note this package needs to be loaded after listings" in this case (maybe via https://www.latex-project.org/help/documentation/lthooks-doc.pdf?). I'd like to avoid these strong ordering for every new package we start supporting.

But one might also argue, we could also investigate how to avoid the strict ordering in the case of hyperref (with \hypersetup it might/should be a quite similar case here) which would make this "feature" a more "global" one deserving its own PR.

[atticus-sullivan]

Note, I intentionally did not push the updated assets and examples yet to avoid frequent changes to binary files in git