Parce

- improvements in Python Console parsing
- fixes and improvements in LilyPond language parsing
- MutableString factored out of Document
- Document: undo/redo uses original changes, better preserves cursor positions

- added LilyPond 2.23 syntax
- remove some cruft to make API cleaner and somewhat smaller
- make sure ``text in context`` or ``lexicon in context`` works in Python >=3.9
(issue 1)
- documentation improvements

- added test module for lang.numbers
- added Nederlands, Francais and Deutsch to lang.numbers

- added lang.numbers to parse english numbers
- lexer: only advance position in circular target when pushing
- scheme: fixed shorthand complex notations like 0+i, -i etc.
- lilypond: recognize $ and as Scheme starters
- small improvements and documentation improvements

- general code cleanup and improvements
- transform: add Items.arg, so a transform can access the lexicon argument
- lilypond language definition:
* fixed whitespace in list separator token
* added \fermata markup command
* unit now has its own action Name.Builtin.Unit
- scheme language definition:
* better number parsing, now supports all features of Scheme/Guile numbers
bin/dec/hex/oct, fraction, +/- inf, nan, complex, polar, exact/inexact
* a helper function parsing scheme number tokens into a python value
* test suite for scheme numbers

- small change in GroupToken handling: now the last token has a negative index.
This makes it easier to recognize the last group token in a stream.
- adapt some more places where erroneously Python 3.8 syntax was used (asterisk
in return or yield statements without parentheses).
- lilypond language definition:
* in lyric word, highlight tie and space separately
* added many missing commands/words