LibJS: Add the ToTemporalInstant Abstract Operation & its requirements

This is Abstract Operation is required for the majority of
InstantConstructor's and InstantPrototype's methods.

The implementation is not entirely complete, (specifically 2 of the
underlying required abstract operations, ParseTemporalTimeZoneString
and ParseISODateTime are missing the required lexing, and as such are
TODO()-ed) but the majority of it is done.

Userland/Libraries/LibJS/Runtime/Temporal/Calendar.cpp

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/Temporal/Calendar.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS::Temporal {
+
+// 12.1.30 IsISOLeapYear ( year ), https://tc39.es/proposal-temporal/#sec-temporal-isisoleapyear
+bool is_iso_leap_year(i32 year)
+{
+    // 1. Assert: year is an integer.
+
+    // 2. If year modulo 4 ≠ 0, return false.
+    if (year % 4 != 0)
+        return false;
+
+    // 3. If year modulo 400 = 0, return true.
+    if (year % 400 == 0)
+        return true;
+
+    // 4. If year modulo 100 = 0, return false.
+    if (year % 100 == 0)
+        return false;
+
+    // 5. Return true.
+    return true;
+}
+
+// 12.1.32 ISODaysInMonth ( year, month ), https://tc39.es/proposal-temporal/#sec-temporal-isodaysinmonth
+i32 iso_days_in_month(i32 year, i32 month)
+{
+    // 1. Assert: year is an integer.
+
+    // 2. Assert: month is an integer, month ≥ 1, and month ≤ 12.
+    VERIFY(month >= 1 && month <= 12);
+
+    // 3. If month is 1, 3, 5, 7, 8, 10, or 12, return 31.
+    if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12)
+        return 31;
+
+    // 4. If month is 4, 6, 9, or 11, return 30.
+    if (month == 4 || month == 6 || month == 9 || month == 11)
+        return 30;
+
+    // 5. If ! IsISOLeapYear(year) is true, return 29.
+    if (is_iso_leap_year(year))
+        return 29;
+
+    // 6. Return 28.
+    return 28;
+}
+
+}