Add support for the :silent command in the REPL

[Gedochao]

The :silent command was (and is) supported by the Scala 2 REPL, while it is visibly absent in Scala 3.

scala -S 2.13
Welcome to Scala 2.13.15 (OpenJDK 64-Bit Server VM, Java 17).
Type in expressions for evaluation. Or try :help.
scala> :help
All commands can be abbreviated, e.g., :he instead of :help.
(...)
:silent                  disable/enable automatic printing of results
(...)
scala> val loud = "this will get printed"
val loud: String = this will get printed

scala> :silent

scala> val silent = "this won't"