LibLocale+LibJS: Port locale parsing and processing to String

In order to prevent this commit from having to refactor almost all of
Intl, the goal here is to update the internal parsing/canonicalization
of locales within LibLocale only. Call sites which are already equiped
to handle String and OOM errors do so, however.

Userland/Libraries/LibLocale/DateTimeFormat.cpp

@@ -109,7 +109,7 @@ static auto find_regional_values_for_locale(StringView locale, GetRegionalValues
 
     auto return_default_values = [&]() { return get_regional_values("001"sv); };
 
-    auto language = parse_unicode_language_id(locale);
+    auto language = parse_unicode_language_id(locale).release_value_but_fixme_should_propagate_errors();
     if (!language.has_value())
         return return_default_values();