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.cpp

@@ -308,4 +308,59 @@ i32 number_of_months_since_unix_epoch(YearAndMonth year_and_month)
     return (year_and_month.year - 1970) * 12 + year_and_month.month - 1;
 }
 
+// https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#parse-a-week-string
+Optional<WeekYearAndWeek> parse_a_week_string(StringView input_view)
+{
+    // 1. Let input be the string being parsed.
+    // 2. Let position be a pointer into input, initially pointing at the start of the string.
+    GenericLexer input { input_view };
+
+    // 3. Collect a sequence of code points that are ASCII digits from input given position. If the collected sequence is
+    //    not at least four characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer.
+    //    Let that number be the year.
+    auto year_string = input.consume_while(is_ascii_digit);
+    if (year_string.length() < 4)
+        return {};
+    auto maybe_year = year_string.to_number<u32>();
+    if (!maybe_year.has_value())
+        return {};
+    auto year = maybe_year.value();
+
+    // 4. If year is not a number greater than zero, then fail.
+    if (year < 1)
+        return {};
+
+    // 5. If position is beyond the end of input or if the character at position is not a U+002D HYPHEN-MINUS character, then
+    //    fail. Otherwise, move position forwards one character.
+    if (!input.consume_specific('-'))
+        return {};
+
+    // 6. If position is beyond the end of input or if the character at position is not a U+0057 LATIN CAPITAL LETTER W character
+    //    (W), then fail. Otherwise, move position forwards one character.
+    if (!input.consume_specific('W'))
+        return {};
+
+    // 7. Collect a sequence of code points that are ASCII digits from input given position. If the collected sequence is not
+    //    exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that
+    //    number be the week.
+    auto week_string = input.consume_while(is_ascii_digit);
+    if (week_string.length() != 2)
+        return {};
+    auto week = week_string.to_number<u32>().value();
+
+    // 8. Let maxweek be the week number of the last day of year year.
+    auto maxweek = week_number_of_the_last_day(year);
+
+    // 9. If week is not a number in the range 1 ≤ week ≤ maxweek, then fail.
+    if (week < 1 || week > maxweek)
+        return {};
+
+    // 10. If position is not beyond the end of input, then fail.
+    if (!input.is_eof())
+        return {};
+
+    // 11. Return the week-year number year and the week number week.
+    return WeekYearAndWeek { year, week };
+}
+
 }