LibJS: Begin implementing GlobalEnvironmentRecord

These represent the outermost scope in the environment record
hierarchy. The spec says they should be a "composite" of two things:

- An ObjectEnvironmentRecord wrapping the global object
- A DeclarativeEnvironmentRecord for other declarations

It's not yet clear to me how this should work, so this patch only
implements the first part, an object record wrapping the global object.

Userland/Libraries/LibJS/Runtime/GlobalEnvironmentRecord.h

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/EnvironmentRecord.h>
+
+namespace JS {
+
+class GlobalEnvironmentRecord final : public EnvironmentRecord {
+    JS_OBJECT(GlobalEnvironmentRecord, EnvironmentRecord);
+
+public:
+    explicit GlobalEnvironmentRecord(GlobalObject&);
+
+    virtual Optional<Variable> get_from_environment_record(FlyString const&) const override;
+    virtual void put_into_environment_record(FlyString const&, Variable) override;
+    virtual bool delete_from_environment_record(FlyString const&) override;
+    virtual bool has_this_binding() const final { return true; }
+    virtual Value get_this_binding(GlobalObject&) const final;
+
+    Value global_this_value() const;
+
+    // [[ObjectRecord]]
+    ObjectEnvironmentRecord& object_record() { return *m_object_record; }
+
+    // [[DeclarativeReco rd]]
+    DeclarativeEnvironmentRecord& declarative_record() { return *m_declarative_record; }
+
+private:
+    virtual bool is_global_environment_record() const override { return true; }
+    virtual void visit_edges(Visitor&) override;
+
+    GlobalObject& m_global_object;
+
+    ObjectEnvironmentRecord* m_object_record { nullptr };
+    DeclarativeEnvironmentRecord* m_declarative_record { nullptr };
+};
+
+template<>
+inline bool Object::fast_is<GlobalEnvironmentRecord>() const { return is_global_environment_record(); }
+
+}