AsciiFoldingFilter should support vulgar fractions

[michaelrhanson]

Unicode range 0x00BC to 0x00BE (1/4, 1/2, 3/4) and Unicode range 0x2150 to 0x215E (various other fractions), as well as 0x2189 (zero thirds) show up in actual text with some frequency.

It would be helpful if AsciiFoldingFilter supported these, probably with an expansion to a pair of simple ASCII integers (0-9) separated by slash/solidus ('/', ASCII 2F).

See https://github.com/unicode-org/icu/blob/2dce62892b6b00df02195a35ca09ebd8e082e80f/icu4c/source/data/translit/Latin_ASCII.txt for reference.