Add Lilypond support

[alerque]

Add package with commands to embed Lilypond code either inline or as external files that is engraved externally before being included in SILE for layout.

[alerque]

Note that this will be conceptually similar to adding MathJax support (#220) in that a block of content will be passed out to some other program to be rendered and the result embedded in the output.

[simoncozens]

Can't you do this yourself? ;-) I think there's an example of using the inputfilter package for this kind of thing.

[alerque]

Yes I think I can wire this up, I just haven't had the necessary combination of time and motivation. I'm sure the next time I have to fix something in one of my PDF build pipelines that still uses LaTeX for the Lilypond support I'll get in gear and make this happen. Note I'd already self-assigned this issue.

[alerque]

Progress report: I have basic systems rendering in a sile document. Cutting through the hack jobs surrounding the TeX implementations of this is crazy hard but this is going to be a lot easier it SILE than it ever was in TeX!

[alerque]

Progress report: now detecting some basic layout parameters like font size, line width, indent, ragged vs. flush formatting, etc. from the SILE context and passing them to Lilypond.

[alerque]

It's probably worth reviewing the lyluatex project for pointers in implementing this.

[Omikhleia]

This issue (and related PR #499 should be moved to lilypond.sile and closed here as "rejected - to be handled in a 3rd-party package].
(Noting in passing: I'd be interested in Lilypond support to be resurrected there...)

[ctrlcctrlv]

"rejected - to be handled in a 3rd-party package"