Allow setting another "current module" than Main

[KristofferC]

When working on e.g. a package it can sometimes be nice work inside that package module, in other words, the expression you type are evaluated into that module. Both Juno and VSCode has functionality where you can set the "current active module" to something else than Main. Perhaps there could be some hook where a user can set the module code is evaluated to something else.

Should be fairly easy to implement, just change out the hardcoded Mains with some global variable that can be changed.

What do you think?