#55 chore(ci): allow running release-please manually

Adds a workflow_dispatch trigger to the Release Please workflow so it can be run from the Actions tab.

The push event for #54's merge (0d270e7) was never delivered during GitHub's 21 Oct webhook/Actions incident, so release-please never ran and no release PR was opened. Merging this pushes to main and runs release-please, which picks up #54. Next time, it can be started by hand instead.

chore(ci) touches only .github/, so it doesn't bump either package.