Introduce a SERVER platform (tag token `server`) and a third release
scope alongside All / Targeted. A server-only release produces a
`vX.Y.Z+server` tag, which matches none of the per-store publish jobs in
publish-release.yml, so no client app store upload runs while the server
distribution still builds and deploys out of band.
isPlatformReleaseTag now recognizes `+server`, so backout/revert clean it
up like any other release tag.
We've run into many problems where everything publishes fine, except 1 store. This provides a way to unblock that one store without pushing to the rest.