fix: use .cjs extension for CommonJS files (#53)

Author: web-padawan

.gulp.json

@@ -0,0 +1,5 @@
+{
+  "flags": {
+    "gulpfile": "gulpfile.cjs"
+  }
+}

check.js renamed to check.cjs

@@ -8,7 +8,7 @@
     "import './vaadin-usage-statistics-collect.js';"
   ],
   "to": [
-    "\"license\": \"Apache-2.0\",\n\"scripts\": {\n\"postinstall\": \"node check.js\",\n\"disable\": \"npm config set @vaadin/vaadin-usage-statistics:disabled true && npm run postinstall\",\n\"enable\": \"npm config set @vaadin/vaadin-usage-statistics:disabled false && npm run postinstall\"\n},",
+    "\"license\": \"Apache-2.0\",\n\"scripts\": {\n\"postinstall\": \"node check.cjs\",\n\"disable\": \"npm config set @vaadin/vaadin-usage-statistics:disabled true && npm run postinstall\",\n\"enable\": \"npm config set @vaadin/vaadin-usage-statistics:disabled false && npm run postinstall\"\n},",
     "export * from './vaadin-usage-statistics-collect.js';"
   ]
 }