LibJS: Move [[Fields]] to ECMAScriptFunctionObject

Userland/Libraries/LibJS/Runtime/ECMAScriptFunctionObject.h

@@ -58,6 +58,16 @@ public:
     Object* home_object() const { return m_home_object; }
     void set_home_object(Object* home_object) { m_home_object = home_object; }
 
+    struct InstanceField {
+        StringOrSymbol name;
+        ECMAScriptFunctionObject* initializer { nullptr };
+
+        void define_field(VM& vm, Object& receiver) const;
+    };
+
+    Vector<InstanceField> const& fields() const { return m_fields; }
+    void add_field(StringOrSymbol property_key, ECMAScriptFunctionObject* initializer) { m_fields.empend(property_key, initializer); }
+
 protected:
     virtual bool is_strict_mode() const final { return m_strict; }
 
@@ -77,6 +87,7 @@ private:
     ThisMode m_this_mode { ThisMode::Global };                    // [[ThisMode]]
     bool m_strict { false };                                      // [[Strict]]
     Object* m_home_object { nullptr };                            // [[HomeObject]]
+    Vector<InstanceField> m_fields;                               // [[Fields]]
     bool m_is_class_constructor { false };                        // [[IsClassConstructor]]
 
     FlyString m_name;