fix #23369

Adds support for "/" within chordnames if the portion after the "/" is
not convertible to tpc and hence not a valid bass note.  This allows for
C6/9 to be rendered as such.  I look at the *last* slash when deciding
how to proceed, so C6/9/E is handled correctly.
Merge pull request #524 from MarcSabatella/23369-slash-in-chord

fix #23369