Add Scalafmt for mill (backport #2805) (#2807)

* Add Scalafmt for mill (#2805)

* add scalafmt for test in mill build system.
* reformat codes in integration-tests

(cherry picked from commit d881fd29

)

# Conflicts:
#	build.sc
#	integration-tests/src/test/scala/chiselTests/util/experimental/BitPat.scala

* Resolve backport conflicts

Co-authored-by: Jiuyang Liu <liu@jiuyang.me>
Co-authored-by: Jack Koenig <koenig@sifive.com>