Maintenance: light extension to handle automatic spec revbump

[mudler]

We already have the code for it. It just need to be split into a separate extension on its own: https://github.com/Luet-lab/extensions/blob/master/extensions/autobump-github/luet-autobump-github#L254

This would ease out manual revbumps and manual intervention to specs

[mudler]

Afterwards, we can just drop https://github.com/mudler/luet/blob/master/cmd/tree/bump.go from the core. as wouldn't be needed anymore

[chrisdebian]

Opened a fix: Luet-lab/extensions#13

reverse_bump() is now its own extension (luet-revbump-github), runnable standalone against a single package as well as from the autobump loop's revdep sweep. One correction along the way: template_string() turned out not to be a dependency of reverse_bump() after all — it's used separately for the checksum hook path.