Add yarn, just scripts to build extension

This requires some extra manual copying because parcel doesn't yet
understand the manifest format. The as-yet-open pull request
https://github.com/parcel-bundler/parcel/pull/4313 seems like it
should handle this.

extension/package.json

@@ -12,5 +12,8 @@
   "license": "UNLICENSED",
   "devDependencies": {
     "parcel": "^1.12.4"
+  },
+  "scripts": {
+      "build-dist": "parcel build src/background/page.html src/options/options.html src/popup/popup.html"
   }
 }

justfile

@@ -4,3 +4,8 @@ copy-to-pier:
 
 build-javascript:
     yarn --cwd gall-app run build-development
+
+build-extension:
+    yarn --cwd extension run build-dist
+    cp -r extension/src/icons/ extension/dist/
+    cp extension/src/manifest.json extension/dist/