Feature request: Add `preview` releases

[gorillamoe]

Add the possibility to add preview releases.

This is to test out / mimic functionality that relies on releasing, like CI itself, automated downloads and so on. This also makes it easy to have people test out new stuff, without interfering with "stable" releases.

The preview releases should contain the -preview suffix. So a valid preview release can look like this: v3.6.2-preview.