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

One point to note is that I am not entirely sure what the result
of the pre-existing valueAsNumber test should be for this strange
case which does not lie exactly on a week/day boundary. Chrome
gives a negative timestamp, which seems more wrong than the result
we give, and neither gecko or WebKit appear to support the 'week'
type. So I'm considering this result acceptable for now, and this
may be something that will need more WPT tests added in the future.

Libraries/LibWeb/HTML/Dates.h

@@ -28,6 +28,13 @@ struct YearAndMonth {
     u32 month;
 };
 Optional<YearAndMonth> parse_a_month_string(StringView);
+
+struct WeekYearAndWeek {
+    u32 week_year;
+    u32 week;
+};
+Optional<WeekYearAndWeek> parse_a_week_string(StringView);
+
 i32 number_of_months_since_unix_epoch(YearAndMonth);
 
 }