maint(core basepattern): Avoid code linting problem.

Avoid code linting problem and make clear the init method is/can be
asynchronous.

Author: thet

src/core/basepattern.js

@@ -69,7 +69,7 @@ class BasePattern {
         }, 0);
     }
 
-    init() {
+    async init() {
         // Extend this method in your pattern.
     }