Provide a way to distinguish between dev-only releases and dev release of a pre-release

[brettcannon]

As this question has come up a couple of times now I created such discussion: https://discuss.python.org/t/are-developmental-releases-a-type-of-pre-release/81666

If it's decided that dev-releases are a form of pre-release then one thing missing from packaging's current API is a way to distguish between a dev-release of a final release, and a dev-release of a pre-release, e.g. 1.0dev1 and 1.0a1dev1.

Originally posted by @notatallshaw in #875