Implement Engines support

[bopm]

This PR implements support for Rails Engines Tailwind styles. It does not require configuration, but sets basic conventions and allows users to redefine engine styles in the host application.

[flavorjones]

Thanks for thinking about this!

I'd like to land v4 support before I think about what's next, so it may be a day or two before I can review and give good feedback.

[bopm]

Totally, this needs v4 to work. Ping me when you get to this, I am happy to add missing parts including tests.

[bopm]

@flavorjones any updates for your expectations here?

[flavorjones]

@bopm I appreciate your patience. Reviewing this PR is near, but not at, the top of my list and this is going to be a very busy week for me.

[bopm]

Sure, once again, take your time. I only keeping hand on a pulse to be sure it's not blocked by you possibly waiting on something from me.