doc: Implement `@lilycode` to print LilyPond code

The idea is to have an environment for highlighted LilyPond code that is not executed, and that can also contain invalid or rudimentary code.

If lilypond-book's option --highlight is active, code within @lilycode gets coloured, using the @pygments environment or the @code command. Without highlighting, the @verbatim environment or the @code command is used. The reason for using @verbatim and not @example is that the former allows output without indentation.

This is an internal feature, to be used for LilyPond's Texinfo documentation only. For this reason, it is not documented in the Usage Manual.