Have New projects require to Opt-in before they can be shipped as stable

Today, the default is for projects to ship as stable when we enable stable versioning on the repo, and since we ship releases each month, this could potentially mean that we could unintentionally release a brand new package as stable without having it as preview for a few releases first to get some bake time. After discussing with several folks, this issue will track the work for having the default be for packages to ship as preview, and requiring them to opt-in to be marked as stable.