Ensure built assets before releasing

[jtpio]

This repo differs from other extensions and the extension cookiecutter since it does not depend on hatch-jupyter-builder.

hatch-jupyter-builder as a way to make sure some assets are correctly built via ensured-targets in tool.hatch.build.hooks.jupyter-builder: https://github.com/jupyterlab/hatch-jupyter-builder

We should look into having something similar.