Feature request: Ability to launch repl for a script with experimental import parser

Right now, there's a wonderful script parser implemented as a part of https://github.com/commercialhaskell/stack/issues/2805 which lets you write scripts like this:

```haskell
#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
import Control.Concurrent.Async.Lifted
import Control.Monad.State.Strict

main :: IO ()
main = execStateT
(concurrently (modify (+ 1)) (modify (+ 2)))
4 >>= print
```

It would be super-nice to be able to also easily launch a REPL, which parses imports and loads the file.

Start by reading the script-parser context in issue #2805, then locate Stack's existing script and REPL entry points. Determine how a command could launch a REPL that parses imports and loads the selected file; done means the requested script example can be opened in that REPL.

Written by the indexing model from the issue text.