Partial Fix #21242: Add REPL init script setting

[noti0na1]

Partial fix #21242

Test with flag at REPL startup:

> ./bin/scalaQ --repl-init-script 'println("Hello from init script!"); val i = 2 * 2'
[warning] MainGenericRunner class is deprecated since Scala 3.5.0, and Scala CLI features will not work.
[warning] Please be sure to update to the Scala CLI launcher to use the new features.
[warning] Check the Scala 3.5.0 release notes to troubleshoot your installation.
Hello from init script!
val i: Int = 4
Welcome to Scala 3.6.4-RC1-bin-SNAPSHOT-nonbootstrapped-git-5ea7c13 (17.0.12, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                                            
scala> 

Test inside REPL:

sbt:scala3> repl
Welcome to Scala 3.6.4-RC1-bin-SNAPSHOT-nonbootstrapped-git-20e6f11 (17.0.12, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                                            
scala> :reset --repl-init-script:'println("Hello from init script!")'
Resetting REPL state with the following settings:
  --repl-init-script:println("Hello from init script!")

Hello from init script!
                                                                                                                                            
scala> 

scala-cli can use this flag to passing init code to REPL.

[Gedochao]

LGTM

Note that in the current state, this PR covers for the first point in #21242 requirements, only- that is, a way to pass a code snippet or source file to the REPL main class to be executed at REPL startup.

We'll still need a follow-up, to cover for the second one: a way to skip the interactive mode and execute the script inside of the REPL, similar to what the -e flag does outside of the REPL.

Seems to me like we shouldn't close the issue with this just yet.