feat: option to stack commits instead of amending #282

Updates PRs can stay open for more than one cycle

Currently, the update commits are amended

If an update commit from cycle 1 passes but an update commit from cycle 2 fails, that information would be lost by amending

An option to stack commits instead of amending them would be optimal

However, the https://github.com/peter-evans/create-pull-request action (used as a dependency in this action) is not compatible with this behavior (yet, see peter-evans/create-pull-request#4161 (comment)) but the native gh pr create is