LibJS: Start implementing Date :^)

This adds:

- A global Date object (with `length` property and `now` function)
- The Date constructor (no arguments yet)
- The Date prototype (with `get*` functions)

Libraries/LibJS/Runtime/Date.h

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2020, Linus Groh <mail@linusgroh.de>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <LibCore/DateTime.h>
+#include <LibJS/Runtime/Object.h>
+
+namespace JS {
+
+class Date final : public Object {
+public:
+    Date(Core::DateTime datetime, u16 milliseconds);
+    virtual ~Date() override;
+
+    Core::DateTime& datetime() { return m_datetime; }
+    u16 milliseconds() { return m_milliseconds; }
+    virtual Value value_of() const override
+    {
+        return Value(static_cast<double>(m_datetime.timestamp() * 1000 + m_milliseconds));
+    }
+
+private:
+    virtual bool is_date() const final { return true; }
+    virtual const char* class_name() const override { return "Date"; }
+
+    Core::DateTime m_datetime;
+    u16 m_milliseconds;
+};
+
+}