Add `tooltip` to `QuillCustomButton`

lib/src/models/themes/quill_custom_button.dart

@@ -1,11 +1,18 @@
 import 'package:flutter/material.dart';
 
 class QuillCustomButton {
-  const QuillCustomButton({this.icon, this.onTap});
+  const QuillCustomButton({
+    this.icon,
+    this.onTap,
+    this.tooltip,
+  });
 
   ///The icon widget
   final IconData? icon;
 
   ///The function when the icon is tapped
   final VoidCallback? onTap;
+
+  /// The button tooltip.
+  final String? tooltip;
 }