Fix CaseMapping generation script to not depend on GHC's Unicode data

[Lysxia]

The toUpper test failed again

          t_toUpper_char:                                                  FAIL
            *** Failed! Falsified (after 83 tests):
            '\93895'
            "\93895" /= "\93868"
            Use --quickcheck-replay="(SMGen 3510331914146184157 11837289022238670735,82)" to reproduce.
            Use -p '/t_toUpper_char/' to rerun this test only.

(full log: https://github.com/haskell/text/actions/runs/21096966056/job/60675626815)

Our scripts are tied to the GHC version because they use toLower/toUpper/toTitle from Data.Char, and the regenerate.sh script explicitly requires ghc-9.12, so the update to Unicode 17 was incomplete.

The fix is to get a fresh UnicodeData.txt instead of relying on Data.Char.toUpper/toLower/toTitle.

Some of the changes in SpecialCasing.hs also move some characters: they move from the usual section to unusual, to avoid having to do lookup in twiddle (previously toUpper/toLower/toTitle) which are either a bit expensive or annoying to do right.