fix(components): [message-box] ElMessageBoxOptions type missing modalClass (#20135)

fix(components): [MessageBox]ElMessageBoxOptions type add modalClass

packages/components/message-box/src/message-box.type.ts

@@ -84,6 +84,9 @@ export interface ElMessageBoxOptions {
   /** Custom inline style for MessageBox */
   customStyle?: CSSProperties
 
+  /** modal class name for MessageBox */
+  modalClass?: string
+
   /** MessageBox closing callback if you don't prefer Promise */
   callback?: Callback