LibJS: Reorganize ExecutionContext a little bit

- Move it to a separate header file
- Annotate the members that represent spec slots
- Reorganize the members (by spec vs non-spec)

Userland/Libraries/LibJS/Runtime/ExecutionContext.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2020-2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2020-2021, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/FlyString.h>
+#include <LibJS/Forward.h>
+#include <LibJS/Runtime/MarkedValueList.h>
+#include <LibJS/Runtime/Value.h>
+
+namespace JS {
+
+// 9.4 Execution Contexts, https://tc39.es/ecma262/#sec-execution-contexts
+struct ExecutionContext {
+    explicit ExecutionContext(Heap& heap)
+        : arguments(heap)
+    {
+    }
+
+    FunctionObject* function { nullptr };          // [[Function]]
+    Realm* realm { nullptr };                      // [[Realm]]
+    Environment* lexical_environment { nullptr };  // [[LexicalEnvironment]]
+    Environment* variable_environment { nullptr }; // [[VariableEnvironment]]
+
+    ASTNode const* current_node { nullptr };
+    FlyString function_name;
+    Value this_value;
+    MarkedValueList arguments;
+    Object* arguments_object { nullptr };
+    bool is_strict_mode { false };
+};
+
+}