A GitHub Action workflow to update submodules

[lemonsaurus]

As I recently learned, submodules are not kept up to date automatically. We have to update them by running git submodule update --recursive --remote and then committing the result - but that doesn't mean we can't automate this. Let's create a workflow to do the submodule update, and include it in the release workflow. That way, we will automatically update Bulma every time we do a release.