LibWeb/HTML: Implement <input> element valueAsNumber for 'month'

Author: https://github.com/shannonbooth
Commit: 6c2ad49ed2
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3485
Reviewed-by: https://github.com/AtkinsSJ ✅
Reviewed-by: https://github.com/gmta
Reviewed-by: https://github.com/trflynn89

Libraries/LibWeb/HTML/Dates.h

@@ -23,4 +23,11 @@ String normalize_local_date_and_time_string(String const& value);
 bool is_valid_time_string(StringView value);
 WebIDL::ExceptionOr<GC::Ref<JS::Date>> parse_time_string(JS::Realm& realm, StringView value);
 
+struct YearAndMonth {
+    u32 year;
+    u32 month;
+};
+Optional<YearAndMonth> parse_a_month_string(StringView);
+i32 number_of_months_since_unix_epoch(YearAndMonth);
+
 }