LibJS: Don't leak class field initializers

We were storing these in Handle (strong GC roots) hanging off of
ECMAScriptFunctionObject which effectively turned into world leaks.

Libraries/LibJS/Runtime/ECMAScriptFunctionObject.cpp

@@ -544,6 +544,7 @@ void ECMAScriptFunctionObject::visit_edges(Visitor& visitor)
     visitor.visit(m_bytecode_executable);
 
     for (auto& field : m_fields) {
+        visitor.visit(field.initializer);
         if (auto* property_key_ptr = field.name.get_pointer<PropertyKey>(); property_key_ptr && property_key_ptr->is_symbol())
             visitor.visit(property_key_ptr->as_symbol());
     }