Nollup support

[wyozi]

Nollup is a generally faster development-only version of rollup. Supporting nollup if it exists in the package dependencies could also make running tests a bit faster.

[bahmutov]

It does look good, how would I use it though? Can it be called with the same module API? I would love to accept a PR, maybe it could be an option

[wyozi]

I'm not sure how closely the internals match rollup, but at least command line API and plugins should be the same.