add "yarn clean" script

To be safe. Per https://github.com/thierryc/Next-gh-page-example

Author: pwfisher

explorer/package.json

@@ -8,7 +8,8 @@
     "dev": "next",
     "build": "next build && next export",
     "start": "next start",
-    "deploy": "yarn build && touch out/.nojekyll && gh-pages -d out",
+    "clean": "rm -rf node_module/.cache && rm -rf out",
+    "deploy": "yarn clean && yarn build && touch out/.nojekyll && gh-pages -d out",
     "type-check": "tsc"
   },
   "dependencies": {