Move `.mill.sc` extension to `.mill.scala` #148

Job	Run time
test-docs	3m 31s
build-linux / run	3m 39s
build-windows / run	5m 7s
compiler-bridge / run	1m 12s
lint-autofix / run	4m 1s
itest (11, ci/test-mill-dev.sh && ci/test-mill-release.sh && ./mill -i -k __.ivyDepsTree && ./mil... / run	3m 26s
itest (17, ci/test-mill-bootstrap.sh) / run	9m 3s
linux (11, '{scalajslib,scalanativelib}.__.testCached') / run	9m 53s
linux (11, 'example.{depth,extending}.__.local.testCached') / run	8m 56s
linux (11, 'example.thirdparty[{mockito,acyclic,commons-io}].local.testCached') / run	4m 13s
linux (11, 'integration.{failure,feature,ide}.__.local.testCached') / run	9m 54s
linux (17, '{main,scalalib,testrunner,bsp,testkit}.__.testCached') / run	8m 48s
linux (17, 'example.javalib.__.local.testCached') / run	9m 42s
linux (17, 'example.scalalib.__.local.testCached') / run	9m 53s
linux (17, 'example.thirdparty[{fansi,jimfs,netty,gatling}].local.testCached') / run	5m 49s
linux (17, 'integration.invalidation.__.server.testCached') / run	9m 40s
linux (17, 'integration.invalidation.__.fork.testCached') / run	9m 55s
linux (17, contrib.__.testCached) / run	5m 31s
windows (11, contrib.__.testCached) / run	7m 22s
windows (11, "{main,scalalib,bsp}.__.testCached") / run	8m 11s
windows (11, 'integration.invalidation[_].server.testCached') / run	8m 14s
windows (11, "example.scalalib.{basic,web}.__.fork.testCached") / run	8m 35s
windows (17, 'integration.{feature,failure}[_].fork.testCached') / run	8m 21s
	2h 42m 56s

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