Lilypond Treesitter mode for Emacs

Hi all,

I've been working for a few months on a Lilypond major mode for Emacs using
Treesitter: https://github.com/shevvek/lilypond-ts-mode. I had mentioned
this in issue 6743 earlier, but it's now far enough along that it feels
worth sharing on the list.

Currently, lilypond-ts-mode supports:
* Reasonably complete parser-based highlighting for Lilypond code, with
Scheme mode highlighting for embedded Scheme. Most word lists are populated
at runtime.
* Indentation support for arbitrarily nested Scheme and Lilypond code
blocks.
* Geiser implementation with Lilypond as the Guile backend, allowing full
access to the Lilypond runtime Scheme API from Emacs.
* Autodoc support for music functions, markup functions, and Lilypond
exported primitives, usable in both Scheme and Lilypond code.
* Autocompletion support for \-escaped words, and type-aware autocompletion
within Lilypond property expressions.

In the near future I plan to add interactive Lilypond code evaluation and
structured navigation support.

So far, it lacks all the UX niceties of lilypond-mode, like menus,
compilation commands, customization group options, etc.

Hopefully some folks find this interesting/useful! Feedback is welcome.

Saul