Add `.mill.sc` as an alternative file extension to `.mill` #7810

Job	Run time
build-linux / build	50s
build-windows / build	4m 23s
compiler-bridge / build	7s
format-check / build	7s
bincompat-check / build	17s
scalafix-check / build	28s
itest (11, ./mill -i -k __.ivyDepsTree ./mill -i -k __.ivyDepsTree --withRuntime ) / build	15s
itest (11, ci/test-mill-dev.sh) / build	17s
itest (17, ci/test-mill-bootstrap.sh) / build	18s
itest (17, ci/test-mill-release.sh) / build	18s
linux (11, "{main,scalalib,testrunner,bsp}.__.test") / build	8s
linux (11, 'example.depth.__.local.test') / build	8s
linux (11, 'example.thirdparty[{mockito,acyclic,commons-io}].local.test') / build	7s
linux (11, "scalajslib.__.test") / build	8s
linux (11, 'integration.{failure,feature,ide}[_].local.test') / build	8s
linux (11, "scalanativelib.__.test") / build	8s
linux (17, 'example.extending.__.local.test') / build	19s
linux (17, "{main,scalalib,testrunner,bsp}.__.test") / build	7s
linux (17, 'example.javalib.__.local.test') / build	17s
linux (17, 'example.scalalib.__.local.test') / build	7s
linux (17, 'example.thirdparty[{fansi,jimfs,netty,gatling}].local.test') / build	7s
linux (17, 'integration.invalidation[_].fork.test') / build	18s
linux (17, 'integration.invalidation[_].server.test') / build	7s
linux (17, 'integration.invalidation[_].local.test') / build	18s
linux (17, contrib._.test) / build	22s
linux (17, docs.githubPages) / build	8s
windows	1s
publish-sonatype	1s
release-github	1s
	10m 20s

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