Ensure built assets before releasing

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.