Adopt the SMuFL music font encoding standard

I would like to implement support for the SMuFL music notation font standard, in order to give LilyPond access to the growing number of notation fonts that follow it. This is one of the organization's suggestions, but I intend to reverse the order of the listed tasks. First, I will add native support for SMuFL fonts (using the freely available Bravura font as a test case), and then I will make a SMuFL-compliant version of Feta. That way, the core task, which can stand on its own as an optional feature and will benefit LilyPond more, can be completed first in case I don't have time to make Feta SMuFL-compliant.