This change adds tree-sitter-less as an additional dependency, because

we selfishly want to have nice highlighting for LESS files. Most of this

change is infrastructure to add our own layer on top of

tree-sitter-language-pack, letting us extend the list of supported

languages and have our own fallbacks for getting language and parser

objects.

Ran unit tests.
Ran update-language-info and update-queries scripts to generate the

  new data.