Fix major version tag not being updated on release (#31)

The previous versioning workflow used Actions-R-Us/actions-tagger without an
explicit permissions block. With GitHub's read-only default GITHUB_TOKEN it
cannot push the major tag, so `v1` was never moved to v1.3.0.

Replace it with the same script-based workflow used in wearerequired/lint-action:
it declares `contents: write` and force-updates the `vMAJOR` tag to the released
commit via git.