move register_stash from Dist::Zilla::MVP::Assembler::Zilla to Dist::Zil...

...la, for easier access


This will make what I was trying to do in https://github.com/keedi/Pod-Weaver-Section-Contributors/pull/3 and https://github.com/keedi/Pod-Weaver-Section-Contributors/pull/4 more possible -- it would be somewhat silly to expose the $assembler just to be able to access the stashes that are on $zilla anyway.