Add `.mill.sc` as an alternative file extension to `.mill` #7838

Job	Run time
build-windows / build	6m 2s
build-linux / build	4m 17s
format-scalafix-bincompat / build	1m 30s
compiler-bridge / build	1m 42s
itest (17, ci/test-mill-bootstrap.sh) / build	8m 44s
itest (11, ci/test-mill-dev.sh && ci/test-mill-release.sh && ./mill -i -k __.ivyDepsTree && ./mil... / build	3m 20s
linux (11, '{scalajslib,scalanativelib}.__.testCached') / build	10m 18s
linux (11, 'example.{depth,extending}.__.local.testCached') / build	9m 14s
linux (11, 'example.thirdparty[{mockito,acyclic,commons-io}].local.testCached') / build	4m 6s
linux (11, 'integration.{failure,feature,ide}[_].local.testCached') / build	10m 18s
linux (11, docs.githubPages) / build	5m 49s
linux (17, 'example.{javalib,scalalib}.__.local.testCached') / build	10m 22s
linux (17, '{main,scalalib,testrunner,bsp,testkit}.__.testCached') / build	7m 42s
linux (17, 'example.thirdparty[{fansi,jimfs,netty,gatling}].local.testCached') / build	6m 7s
linux (17, 'integration.invalidation[_].{fork,server}.testCached') / build	10m 19s
linux (17, contrib.__.testCached) / build	6m 35s
windows (11, "example.scalalib.{basic,web}.__.fork.test") / build	8m 49s
windows (11, contrib.__.test) / build	8m 21s
windows (11, "{main,scalalib,bsp}.__.test") / build	8m 21s
windows (11, 'integration.invalidation[_].server.test') / build	8m 29s
windows (17, 'integration.{feature,failure}[_].fork.test') / build	8m 32s
publish-sonatype	1s
release-github	1s
	2h 28m 59s

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