Add "yarn clean" to JS bindings

Summary: The build is contained to `build` and `dist` folders we can just wipe.

Reviewed By: huntie

Differential Revision: D42285227

fbshipit-source-id: 18fa4a3e801415169e5b5b8cf212640199e627ba

javascript/just.config.js

@@ -9,6 +9,7 @@
 
 const {
   argv,
+  cleanTask,
   copyTask,
   eslintTask,
   logger,
@@ -27,6 +28,8 @@ const node = process.execPath;
 
 option("fix");
 
+task("clean", cleanTask({paths: ['build', 'dist']}));
+
 task(
   "prepare-for-build",
   parallel(