Primitive tanka CMP

[rassie]

tanka integration seems to be on a wishlist for many people, alongside with pure jsonnet (see #3688, #3124 and probably others). There are a couple of guides on how to make it work without official support.

I've tried to package the information I've seen so far into a tanka configuration management plugin, mostly to make my own life a bit easier -- it's primitive but it works for me. However, I'd very much appreciate the input from the community for the things not yet working but required for the full support and adoption.