Make Resident and object so that its main method can be called from j…

…ava.

We don't usually need it. I played with it to figure out scala#20245, and found out it
was set up incorrectly.

compiler/src/dotty/tools/dotc/Resident.scala

@@ -23,7 +23,7 @@ import scala.annotation.tailrec
  *
  *  dotc> :q     // quit
  */
-class Resident extends Driver {
+object Resident extends Driver {
 
   object residentCompiler extends Compiler