Issue 3555: Parse context definitions and context modifications in \notemode

Issue 3555: Parse context definitions and context modifications in \notemode

Since context modifications and context definitions are somewhat more
concerned with musical content, it makes sense to parse them in a mode
where note names are recognized and can be used in music and scheme
function arguments.

The main argument for a separate INITIAL lexer mode is for being able
to specify REAL number values in a lenient syntax, such as .5\cm.
However, dimensions are almost never used in context definitions
or modifications but rather in output definitions like layout, midi,
paper and header blocks.