refactor!: use utf8proc full casefolding

According to `CaseFolding-15.1.0.txt`, full casefolding should be
preferred over simple casefolding as it's considered to be more correct.
Since utf8proc already provides full casefolding it makes sense to
switch to it. This will also remove a lot of unnecessary build code.

Temporary exceptions are made for two sets characters:

- `ß` will still be considered `ß` (instead of `ss`) as using a full
  casefolding requires interfering with upstream spell files in some
  form.
- `İ` will still be considered `İ` (instead of `i̇`) as using full
  casefolding requires making a value judgement on the "correct"
  behavior. There are two, equally valid case-insensetive comparison for
  this character according to unicode. It is essentially up to the
  implementor to decide which conversion is correct. For this reason it
  might make sense to allow users to decide which conversion should be
  done as an added option to `casemap` in a future PR.

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