LibUnicode: Use BCP 47 data to filter valid numbering system names

There isn't too much of an effective difference here other than that the
BCP 47 data contains some aliases we would otherwise not handle.

Userland/Libraries/LibUnicode/NumberFormat.cpp

@@ -16,7 +16,6 @@
 
 namespace Unicode {
 
-Optional<NumberSystem> __attribute__((weak)) number_system_from_string(StringView) { return {}; }
 Optional<StringView> __attribute__((weak)) get_number_system_symbol(StringView, StringView, NumericSymbol) { return {}; }
 Optional<NumberGroupings> __attribute__((weak)) get_number_system_groupings(StringView, StringView) { return {}; }
 Optional<NumberFormat> __attribute__((weak)) get_standard_number_system_format(StringView, StringView, StandardNumberFormatType) { return {}; }