fix: Use const constructor for EmbedBuilder (singerdmx#1160)

Author: veselv2010

lib/src/widgets/embeds.dart

@@ -6,6 +6,8 @@ import '../models/themes/quill_icon_theme.dart';
 import 'controller.dart';
 
 abstract class EmbedBuilder {
+  const EmbedBuilder();
+
   String get key;
   bool get expanded => true;