[draft] refactor: replace Song::modeNotes and numModeNotes with a NoteSet

[nikodemus]

• NoteSet::operator[] gets note value of a scale degree.
• NoteSet::fromScaleNotes() constructs a NoteSet from scale notes.
• NoteSet::applyChanges() applies an array of scale degree semitone offsets.
• NoteSet::addUntrusted() implments safeguards that loading from song file uses.
• NoteSet::clear() clears a set.
• NoteSet::highest() returns the highest note in the set.

There are no known issues with this patch, but given its widespread nature I want write more comprehensive tests and run it a while before saying it's done. 😅

[github-actions[bot]]

Test Results

54 tests  +8   54 :white_check_mark: +8   0s :stopwatch: ±0s  8 suites ±0    0 :zzz: ±0   8 files   ±0    0 :x: ±0 

Results for commit 43f1ed2e. ± Comparison against base commit 176108a1.

:recycle: This comment has been updated with latest results.

[nikodemus]

2201 is it