Use .js imports in src (#727)

* Use .js in src imports

* Bump dev

package.json

@@ -30,7 +30,7 @@
     "test:one": "polkadot-dev-run-test --env browser"
   },
   "devDependencies": {
-    "@polkadot/dev": "^0.70.4",
+    "@polkadot/dev": "^0.71.1",
     "@polkadot/x-bundle": "^11.0.1",
     "@types/node": "^18.14.6",
     "react": "^18.2.0",