LibJS: Start implementing Temporal.TimeZone

Here we got our first Temporal object :^)
This patch adds the TimeZone object itself, its constructor and
prototype (currently empty), and a bunch of required abstract operations

Userland/Libraries/LibJS/Runtime/Temporal/TimeZone.h

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <LibJS/Runtime/Object.h>
+
+namespace JS::Temporal {
+
+class TimeZone final : public Object {
+    JS_OBJECT(TimeZone, Object);
+
+    // Needs to store values in the range -8.64 * 10^21 to 8.64 * 10^21
+    using OffsetType = double;
+
+public:
+    explicit TimeZone(String identifier, Object& prototype);
+    virtual ~TimeZone() override = default;
+
+    String const& identifier() const { return m_identifier; }
+    Optional<OffsetType> const& offset_nanoseconds() const { return m_offset_nanoseconds; }
+    void set_offset_nanoseconds(u32 offset_nanoseconds) { m_offset_nanoseconds = offset_nanoseconds; };
+
+private:
+    // 11.5 Properties of Temporal.TimeZone Instances, https://tc39.es/proposal-temporal/#sec-properties-of-temporal-timezone-instances
+
+    // [[Identifier]]
+    String m_identifier;
+
+    // [[OffsetNanoseconds]]
+    Optional<OffsetType> m_offset_nanoseconds;
+};
+
+bool is_valid_time_zone_name(String const& time_zone);
+String canonicalize_time_zone_name(String const& time_zone);
+String default_time_zone();
+Object* create_temporal_time_zone(GlobalObject&, String const& identifier, FunctionObject* new_target = nullptr);
+
+}