Staff constructor
Creates a Staff with the given measures list.
lineCount defaults to 5 (standard CMN staff). Set to 1 for percussion,
4 for 4-string tablature, or 6 for guitar tablature.
If measures is omitted an empty list is used, and measures can be
added later via add.
Implementation
Staff({List<Measure>? measures, this.lineCount = 5})
: measures = measures ?? [];