Add caching for the Mill build tool

[clintval]

Description:

Similar to Gradle, Maven, and sbt, I am hoping to see Mill cache support.

Mill is becoming a popular JVM build tool so I hope this is a welcome addition.

I have not contributed to this project before so please let me know if I missed anything!

Speaking of, how do I run automated tests on this branch?

Check list:

• [x] Mark if documentation changes are required.
• [x] Mark if tests were added or updated to cover the changes.