fix: use main-SNAPSHOT as best-effort fallback for jitpack

[maxandersen]

https://github.com/jbangdev/jbang/issues/1817 made me bite the bullet and have github urls default to use main-SNAPSHOT so it wont fail on new stuff. The whole reason I hesitated was that jitpack in past would accept HEAD-SNAPSHOT to pickup whatever is the default name on github (main or master). But that stop working some years ago - back then I hoped it was just a glitch..but since not fixed i'm now at least making it work for new things.

also updated the docs and added more tests as things were slightly incorrect.

Fixes #1817