LibJS: De-duplicate ClassFieldDefinition Records

This was defined twice, despite being the very same thing:
- ClassElement::ClassFieldDefinition
- ECMAScriptFunctionObject::InstanceField

Move the former to a new header and use it everywhere. Also update the
define_field() AO to take a single field instead of separate name and
initializer arguments.

Userland/Libraries/LibJS/Runtime/ClassFieldDefinition.h

@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+#include <LibJS/Forward.h>
+
+namespace JS {
+
+using ClassElementName = Variant<PropertyKey, PrivateName>;
+
+// 6.2.10 The ClassFieldDefinition Record Specification Type, https://tc39.es/ecma262/#sec-classfielddefinition-record-specification-type
+struct ClassFieldDefinition {
+    ClassElementName name;                        // [[Name]]
+    Handle<ECMAScriptFunctionObject> initializer; // [[Initializer]]
+};
+
+}