Fix GH-15087 IntlChar::foldCase()'s $option is not optional

Since that parameter is supposed to be optional (and has been prior to
PHP 8.0.0), we fix the implementation instead of the st

Fix GH-15087 IntlChar::foldCase()'s $option is not optional

Since that parameter is supposed to be optional (and has been prior to
PHP 8.0.0), we fix the implementation instead of the stub.

Closes GH-15091.

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