[stdlib] Add caseFolded to scalar properties

[Azoy]

This add the _caseFolded property to Unicode.Scalar.Properties and hides it behind Unicode SPI.

From the string processing PR:

Michael asks:

Out of curiosity, do we want this as a buffer of scalars or just UTF-8? Also, is there a fast-path check to return self, or is this function call effectively the fast path? If so, then I think it's even more viable to return UTF-8 rather than decoding and re-encoding the scalars.

Note this is a property of Unicode.Scalar.Properties, so we don't have a string self to begin with. The data structure stores distances from the base scalar to each of its mapped scalars. We could store UTF-8 here to avoid the scalar decoding, but using UTF-8 in this current data structure would actually be larger than storing distances (17 bit distances).

[Azoy]

@swift-ci please smoke test