CLI command for reformatting lydown source files

[noteflakes]

We should be able to issue a lydown fmt <path> to automatically reformat a source file or an entire directory of source files.

The formatter should:

• Insert a space between consecutive notes.
• Insert a line break between measures.

[noteflakes]

See also: https://git.openlilylib.org/bfsc/das-trunkne-lied/wikis/LilyPond-Coding-Guidelines

Ideas we could use:

• Tempo, key etc. should have a line of their own.
• Line length should not exceed 80 characters. If that requires line breaks in mid-measure, indent subsequent lines one more level.
• Explicit duration for the first note on every line.
• barcheck at end of each line of music (using |).