Add quickPrePR sbt cmd

Author: japgolly

prePR.sbt

@@ -1,7 +1,10 @@
 import Build._
+import Dependencies.Ver._
 
 addCommandAlias("prePR", "+prePR_nonCross")
 
+addCommandAlias("quickPrePR", s"++$scala212; prePR_nonCross; ++$scala213; prePR_nonCross")
+
 val prePR_nonCross = taskKey[Unit]("Performs all necessary work required before submitting a PR, for a single version of Scala.")
 
 // Unfortunately we can't just call `root/Test/compile` because it doesn't take aggregation into account :(