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/PlainDate.cpp

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/Temporal/Calendar.h>
+#include <LibJS/Runtime/Temporal/PlainDate.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS::Temporal {
+
+// 3.5.5 IsValidISODate ( year, month, day ), https://tc39.es/proposal-temporal/#sec-temporal-isvalidisodate
+bool is_valid_iso_date(i32 year, i32 month, i32 day)
+{
+    // 1. Assert: year, month, and day are integers.
+
+    // 2. If month < 1 or month > 12, then
+    if (month < 1 || month > 12) {
+        // a. Return false.
+        return false;
+    }
+
+    // 3. Let daysInMonth be ! ISODaysInMonth(year, month).
+    auto days_in_month = iso_days_in_month(year, month);
+
+    // 4. If day < 1 or day > daysInMonth, then
+    if (day < 1 || day > days_in_month) {
+        // a. Return false.
+        return false;
+    }
+
+    // 5. Return true.
+    return true;
+}
+
+}