add printTypesAndErasure test main method to compare typer and erasure

[bishabosha]

run in sbt with

scala3> scala3-compiler/Test/runMain
  dotty.tools.printTypesAndErasure
  "class Box { type X }"
  "method"
  "(b: Box)(x: b.X): List[b.X]"`

see output like

[info] running (fork) dotty.tools.printTypesAndErasure "class Box { type X }" method "(x: Box#X): List[Box#X]"
info @ typer   => (b: Box)(x: b.X): scala.collection.immutable.List[b.X]
info @ erasure => (b: Box, x: Object): scala.collection.immutable.List