substitution statefulness

[captaincaius]

Is your feature request related to a problem? Please describe.
In normal vim I often will do a replacement, find the next one, and repeat the replacement with just :s - we currently don't have this

Describe the solution you'd like
state for the substitute, mirroring all the quirks vim has when mixing substitutions and searches in various orders ;)

Describe alternatives you've considered
All you can do is repeat the full :s/search/replace :'(

Additional context
I think the tests I wrote will make it clear ;)

Also... This is somewhat related to #2549 which is a little messy (mostly cause of me - sorry). I'm breaking the various things discussed there into distinct work chunks.