Add a Dependabot config to keep GitHub action versions updated

[kurtmckee]

This is an infrastructure improvement.

The GitHub action versions are no longer current and are issuing deprecation warnings (recent example for actions/checkout@v2, which is now at v4).

Rather than submitting a PR to update the versions once, this PR introduces a Dependabot configuration that will keep the action versions updated in perpetuity.

If this merges, you can expect Dependabot to immediately submit PR(s) to update out-of-date action versions.

Thanks for your work on Bulma!

[kurtmckee]

Note: This supersedes the action version changes introduced in #3628, as Dependabot will update the versions automatically by opening PRs.

The typo fixes in #3628 may or may not still be needed.