lilypond-book: Fix and improve regular expressions to parse input

Nested braces (up to 10 levels) in \lilypond (LaTeX) and @lilypond (Texinfo) are now supported.

Also do the following.

• Beautify some regular expressions to make them more readable.

• Fix some bugs.

  • In HTML, <lilypond .../> and <lilypondfile .../> can now span more than a single line.

  • In LaTeX and Texinfo modes, avoid too greedy scanning of options, which could span up to the closing ] character of a next lilypond command or environment.

  • Ditto for HTML mode, but up to the next : character.

  • The LaTeX template code missed a trailing comment to suppress insertion of whitespace into the printed output.

• Add a regtest.