`@lilycode` for highlighting LilyPond code snippets

[Werner LEMBERG]

Folks,


in MR !1753

  https://gitlab.com/lilypond/lilypond/-/merge_requests/1753

I've extended `lilypond-book` to highlight LilyPond code snippets,
without producing actual LilyPond output.  Because it uses Pygments,
code snippets are also highlighted (more or less) correctly if the
code is wrong, which is sometimes useful to show for documentation
purposes.

Example 1:

```
blabla @lilycode{\relative { c'2 e }} blabla
```

Example 2:

```
blabla

@lilycode
\relative { c'2 e {
@end lilycode
```

While the use for the block command version (`@lilycode ... @end
lilycode`) should be clear, the main question is what policy we are
going to recommend for the inline version: I suggest to apply
`@lilycode` only if highlighting has some useful effects, which means
that in most cases it should be rather *not* used because inline code
is very short.  Has anyone a suggestion for a better formulation, or
better constraints?


    Werner