A new tag for using more than one font in the chant scores

[rpspringuel]

Requested by Sr. Maria Ruth in the MWE of #1444:

Sometimes a different font is needed for rendering a particular situation in the chant scores. For example, to automatically insert a ligature that is not in the main font (e.g. "fí"); the <sp> or <v> tags do not work fine with regard to the vowel detection. This case is similar to a style changing, but it is conceptually different; hence a new tag should be provided (hint: <f></f>) with its own user command(s) in GregorioTeX.