ENH Allow workflow_dispatch event

[emteknetnz]

Issue silverstripe/.github#94

[GuySartorelli]

The commit message is "Allow workflow_dispatch event" but this PR is clearly doing a lot more than that.
I haven't reviewed the rest of the changes yet because it's not clear to me why they're there. What problem are the other changes solving?

Please answer that question for me, and update the commit message to something that covers all the changes in this commit.

[emteknetnz]

The commit message is actually fairly accurate. Previously this action was only running on tag so env variable GITHUB_REF: ${{ github.ref }} was always set to semver tag. When running workflow_dispatch however there is github.ref is not a tag so instead I've updated the action in that scenario to auto-tag based on the 'highest' semver tag according to the github api tags endpoint

[emteknetnz]

@GuySartorelli Have updated code and manually verified on https://github.com/emteknetnz/gha-issue/tags that the updated logic works for both manually created tags (1.0.7, though the v1 tag has since been updated) and automatic tags (called via github-action api by action-ci.yml after it automatically tags a release - 1.0.8 - v1 tag matches)