LibUnicode: Use BCP 47 data to generate available calendars and numbers

BCP 47 will be the single source of truth for known calendar and number
system keywords, and their aliases (e.g. "gregory" is an alias for
"gregorian"). Move the generation of available keywords to where we
parse the BCP 47 data, so that hard-coded aliases may be removed from
other generators.

Userland/Libraries/LibUnicode/NumberFormat.cpp

@@ -16,7 +16,6 @@
 
 namespace Unicode {
 
-Span<StringView const> __attribute__((weak)) get_available_number_systems() { return {}; }
 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 {}; }