[libc][wctype] Implement public iswdigit (#181635)
https://github.com/llvm/llvm-project/issues/175003
Create public function and entrypoints for iswdigit and add UTF8 test to
iswalpha test. On CI there is no UTF8 build currently, so that test case
will only run locally, but
[wctype_utils_test.cpp](https://github.com/llvm/llvm-project/blob/main/libc/test/src/__support/wctype_utils_test.cpp)
is written in a way that both ASCII and UTF8 tests run regardless of the
build config.
