Add repl-init-package setting to configure REPL-entered code package

[noti0na1]

Add --repl-init-package flag to config the REPL code wrapper.

scala>:settings --repl-init-package:my.app
scala> class Foo
// defined class Foo
scala> classOf[Foo].getName
val res0: String = "my.app.rs$line$1$Foo"

How much have you relied on LLM-based tools in this contribution?

moderately

Additional notes

Could be used to avoid the unnamed package issue in CC: #25758

Mainly for testing now. Do not merge.