feat(mill): add Mill plugin #5013

Job	Run time
Test Maven plugin	1m 43s
Test on JDK 17 (windows-latest)	4m 42s
Test on JDK 25 (ubuntu-latest)	2m 49s
Test on JDK 25 (windows-latest)	4m 5s
Test on JDK 17 (ubuntu-latest)	3m 16s
sbt 1 plugin scripted tests (ubuntu-latest)	5m 4s
Mill plugin integration test	3m 32s
sbt 1 plugin scripted tests (windows-latest)	8m 15s
Test formatting	27s
release	0s
	33m 53s

Provide feedback