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/PlainTime.cpp

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/Temporal/PlainTime.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS::Temporal {
+
+// 4.5.5 IsValidTime ( hour, minute, second, millisecond, microsecond, nanosecond ), https://tc39.es/proposal-temporal/#sec-temporal-isvalidtime
+bool is_valid_time(i32 hour, i32 minute, i32 second, i32 millisecond, i32 microsecond, i32 nanosecond)
+{
+    // 1. Assert: hour, minute, second, millisecond, microsecond, and nanosecond are integers.
+
+    // 2. If hour < 0 or hour > 23, then
+    if (hour < 0 || hour > 23) {
+        // a. Return false.
+        return false;
+    }
+
+    // 3. If minute < 0 or minute > 59, then
+    if (minute < 0 || minute > 59) {
+        // a. Return false.
+        return false;
+    }
+
+    // 4. If second < 0 or second > 59, then
+    if (second < 0 || second > 59) {
+        // a. Return false.
+        return false;
+    }
+
+    // 5. If millisecond < 0 or millisecond > 999, then
+    if (millisecond < 0 || millisecond > 999) {
+        // a. Return false.
+        return false;
+    }
+
+    // 6. If microsecond < 0 or microsecond > 999, then
+    if (microsecond < 0 || microsecond > 999) {
+        // a. Return false.
+        return false;
+    }
+
+    // 7. If nanosecond < 0 or nanosecond > 999, then
+    if (nanosecond < 0 || nanosecond > 999) {
+        // a. Return false.
+        return false;
+    }
+
+    // 8. Return true.
+    return true;
+}
+
+}