Better support for fallback from one set of fonts to another.

[pnorman]

In openstreetmap-carto we try to fall back from bold fonts to book fonts with MSS like this

@book-fonts:    "DejaVu Sans Book", "Arundina Sans Regular", "Padauk Regular", "Khmer OS Metal Chrieng Regular", "Droid Sans Fallback Regular", "unifont Medium";
@bold-fonts:    "DejaVu Sans Bold", "Arundina Sans Bold", "Padauk Bold", 
                "DejaVu Sans Book", "Arundina Sans Regular", "Padauk Regular", "Khmer OS Metal Chrieng Regular", "Droid Sans Fallback Regular", "unifont Medium";
@oblique-fonts: "DejaVu Sans Oblique", "Arundina Sans Italic", 
                "DejaVu Sans Book", "Arundina Sans Regular", "Padauk Regular", "Khmer OS Metal Chrieng Regular", "Droid Sans Fallback Regular", "unifont Medium";

This means repeating the @book-fonts list of fonts for both bold and oblique. It would be cleaner to be able to refer to it once. Apparently @bold-fonts: "DejaVu Sans Bold", "Arundina Sans Bold", "Padauk Bold", @book-fonts results in it attempting to find a font with the name of all of the book fonts.

[pnorman]

See also gravitystorm/openstreetmap-carto@4a33288#commitcomment-5084620