feat(mill): add Mill plugin #5022

Job	Run time
Test on JDK 25 (ubuntu-latest)	2m 22s
Test on JDK 25 (windows-latest)	3m 57s
Test formatting	36s
Test on JDK 17 (windows-latest)	4m 30s
Test Maven plugin	2m 24s
sbt 1 plugin scripted tests (windows-latest)	7m 35s
sbt 1 plugin scripted tests (ubuntu-latest)	4m 35s
Test on JDK 17 (ubuntu-latest)	3m 38s
Mill plugin integration test	3m 27s
release	-1s
	33m 3s

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