fix: amend CommonJS output to be compatible with require (#36)

Add `module.exports = module.exports.default;` at the end of the CJS file.
Example:

```js
const ruleset = require('@apisyouwonthate/style-guide');
```

package.json

@@ -54,6 +54,9 @@
     "dts": true,
     "format": ["cjs", "esm"],
     "sourcemap": true,
-    "noExternal": ["@stoplight/types"]
+    "noExternal": ["@stoplight/types"],
+    "footer": {
+      "js": "module.exports = module.exports.default;"
+    }
   }
 }