LibJS: Add getter/setter support

This patch adds a GetterSetterPair object. Values can now store pointers
to objects of this type. These objects are created when using
Object.defineProperty and providing an accessor descriptor.

Libraries/LibJS/Runtime/Accessor.h

@@ -0,0 +1,79 @@
+/*
+ * Copyright (c) 2020, Matthew Olsson <matthewcolsson@gmail.com>
+ * 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.
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/Cell.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS {
+
+class Accessor final : public Cell {
+public:
+    static Accessor* create(Interpreter& interpreter, Value getter, Value setter)
+    {
+        return interpreter.heap().allocate<Accessor>(getter, setter);
+    }
+
+    Accessor(Value getter, Value setter)
+        : m_getter(getter)
+        , m_setter(setter)
+    {
+    }
+
+    Value getter() { return m_getter; }
+    Value setter() { return m_setter; }
+
+    Value call_getter(Value this_object)
+    {
+        if (!getter().is_function())
+            return js_undefined();
+        return interpreter().call(getter().as_function(), this_object);
+    }
+
+    void call_setter(Value this_object, Value setter_value)
+    {
+        if (!setter().is_function())
+            return;
+        MarkedValueList arguments(interpreter().heap());
+        arguments.values().append(setter_value);
+        interpreter().call(setter().as_function(), this_object, move(arguments));
+    }
+
+    void visit_children(Cell::Visitor& visitor) override
+    {
+        visitor.visit(m_getter);
+        visitor.visit(m_setter);
+    }
+
+private:
+    const char* class_name() const override { return "Accessor"; };
+
+    Value m_getter;
+    Value m_setter;
+};
+
+}