Add command line option to stack script to open it in ghci instead of running it

I'm aware that this option exists:

but I tend to want to do this while developing a script and at no other time.

It would be useful to have an option `stack script --ghci` so that I can start a script in GHCi from my editor tooling, but still leave the header the same, so that I can run it normally when I execute it.

I would be happy to implement this if there is interest, and I can get a little bit of help getting started with this repo if I need it.