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

[Gedochao]

The :require 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.
(...)
:require <path>          add a jar to the classpath
(...)
scala> :require cache/https/repo1.maven.org/maven2/com/lihaoyi/os-lib_2.13/0.10.7/os-lib_2.13-0.10.7.jar
Added '/user/test/cache/https/repo1.maven.org/maven2/com/lihaoyi/os-lib_2.13/0.10.7/os-lib_2.13-0.10.7.jar' to classpath.

scala> :require cache/https/repo1.maven.org/maven2/com/lihaoyi/geny_2.13/1.1.1/geny_2.13-1.1.1.jar
Added '/user/test/cache/https/repo1.maven.org/maven2/com/lihaoyi/geny_2.13/1.1.1/geny_2.13-1.1.1.jar' to classpath.

scala> os.pwd
val res0: os.Path = /user/test

scala> 

This is not exactly as good as direct support for //> using dep from inside of the REPL, and would be the equivalent of the //> using jar directive of the runner (that indeed, could be called from inside of the REPL).

[mbovel]

This issue was picked for the Scala Issue Spree of tomorrow, Monday, February 3rd. @dwijnand and @aherlihy will be working on it.

[SethTisue]

I suspect hardly anyone knows that :require even exists in the Scala 2 REPL. afaicr, I've never seen it mentioned.

On the other hand, this could be a nice stepping stone to eventually supporting on-the-fly //> using dep, as that will break down into two steps: calling Coursier to make sure the dependencies are available on the local filesystem, then adding the JARs to the REPL class path.

[SethTisue]

Designwise, I think :require has always been a poor name. It's vague and it's easily confusable with the require method in stdlib.

If this is going in, I'd suggest picking a better name (or even going straight to //> using jar ...), perhaps with :require as a deprecated alias.

[Gedochao]

If this is going in, I'd suggest picking a better name (or even going straight to //> using jar ...), perhaps with :require as a deprecated alias.

+1, although I wouldn't go straight to //> using jar for now (although I'm hoping we'll be able to have //> using jar and //> using dep in the future inside of the REPL).

for now, :jar, perhaps?

[SethTisue]

:jar SGTM