(1/2) Hero Prefetcher Flipper Plugin

Summary: Created new/empty plugin following using scarf: https://www.internalfb.com/intern/staticdocs/flipper/docs/tutorial/intro/#scaffolding-a-new-plugin

Differential Revision: D38415194

fbshipit-source-id: 6e89f4ec59c1ec3e229766044dfcf76f0ce6c9e9

desktop/static/icons.json

@@ -646,6 +646,9 @@
   "nav-magnifying-glass": [
     16
   ],
+  "list-arrow-down": [
+    16
+  ],
   "photo-arrows-left-right": [
     16
   ]