Calculate major and minor semver tags, e.g. for tagging containers

First release of a GitHub action for calculating tags to be used on artifacts where partial major and major.minor tags are also wanted when a major.minor.patch tag is created. As a special case a tag of the form major.minor.patch-N where N is an integer will also be tagged as major.minor.patch, etc. Technically this is a semver pre-release, but is treated as a full release since this is a common way to tag Docker containers.