Add support for the `:save` command in the REPL

[Gedochao]

The :save 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.
(...)
:save <path>             save replayable session to a file
(...)

This would enable coming back to a past session of the REPL along with the (already present) :load command.

[som-snytt]

Sample invisibility:

in comment to

The answer was; "This is possible as of Scala 2.11."