gh-106320: Remove private _PyUnicode C API (#107185)

Move private _PyUnicode functions to the internal C API
(pycore_unicodeobject.h):

* _PyUnicode_IsCaseIgnorable()
* _PyUnicode_IsCased()
* _PyUnicode_IsXidContinue()
* _PyUnicode_IsXidStart()
* _PyUnicode_ToFoldedFull()
* _PyUnicode_ToLowerFull()
* _PyUnicode_ToTitleFull()
* _PyUnicode_ToUpperFull()

No longer export these functions.