Add Binder on PR workflow

[jtpio]

References

Similar setup as the ipywidgets repo: https://github.com/jupyter-widgets/ipywidgets

This will help test changes on Binder without having to check out the code locally.

Code changes

Binder configuration.

User-facing changes

Users and contributors can test their changes on Binder.

Backwards-incompatible changes

None

[jtpio]

The action will add Binder buttons like the following on new PRs:

[jtpio]

This also means we won't be using a released version of voila on Binder in this repo.

But it should be fine since the stable branch will still point to a release commit. The difference will be that voila will be built from source.

We can always quickly setup GitHub gists with voila releases if needed.