Generally follows the same pattern as #22520
There are some changes here, notably it uses the addon's babel
settings rather than the app's, and it goes through the same
treatment as the rest of the addon code (which may include more
than just babel).
However, this probably brings us closer to the normal expectations
you have around developing addon code, and in any case, does not
seem to have any effect on the final output:
```
$ diff dist/assets/markdown-it-bundle.js /tmp/dist-before/assets/markdown-it-bundle.js
```