Allow choosing the notation system language by code

[albertms10]

Currently, a NotationSystem has to be explicitly typed at usage, which is already fine when it is statically known, but not when it should be known at runtime. A more convenient static method would suffice for the purpose.

So, instead of writing:

  formatter: const EnglishNoteNameNotation();

A more flexible API could be:

  formatter: NoteNameNotation.language('en');

Or even:

  formatter: NotationSystems<NoteName>.language('en');

Requiring some static extensions on the type NotationSystems.