fix: remove .cjs extension in favor of .js

`microbundle` only produces the `.cjs` extension if the top-level package uses `"type": "module"`

This was working locally for me only because my `dist` folder has a version generated from trying to swap that but giving up. Thus, CI failed but local testing passed.

package.json

@@ -4,10 +4,10 @@
   "description": "Verify testdouble stubs with QUnit",
   "source": "./src/index.ts",
   "exports": {
-    "require": "./dist/index.cjs",
+    "require": "./dist/index.js",
     "default": "./dist/index.modern.js"
   },
-  "main": "./dist/index.cjs",
+  "main": "./dist/index.js",
   "module": "./dist/index.module.js",
   "unpkg": "./dist/index.umd.js",
   "types": "./dist/index.d.ts",