Create a reusable workflow to commit build assets to dependabot PRs

[SteelWagstaff]

Dependabot PRs sometimes update JS dependencies declared in a repo's package.json. In order for the update to have full impact, we often need to run npm run build in the repo after updating the package.json or package.lock file to rebuild related assets. We should create a GitHub action that automatically checks out the dependabot PR branch, runs npm ci && npm run build and commits any update build assets to the dependabot PR.

In pressbooks/pressbooks, we have a post-commit action for dependabot PRs that autobuilds assets and commits them to the PR: https://github.com/pressbooks/pressbooks/blob/dev/.github/workflows/autobuild-dependabot-assets.yml. This PR doesn't play well with our required 'lint & build assets' step, however. See pressbooks/pressbooks#3206 or screenshot below

It only works now because our required actions are broken and no longer running on our PRs.

Once we have a reusable workflow that does this, we should add it to all the repos where it would be useful.