Message: added type def for offset option (#16027)

types/message.d.ts

@@ -45,6 +45,9 @@ export interface ElMessageOptions {
 
   /** Callback function when closed with the message instance as the parameter */
   onClose?: CloseEventHandler
+  
+  /** Set the distance to the top of viewport. Default is 20 px. */
+  offset?: number
 }
 
 export interface ElMessage {