Implement a REPL

[samaaron]

It would be nice to have a simple REPL available for quick prototyping and noodling which could be toggled with a simple shortcut.

This would allow single/multi-line editing of short code phrases to be executed independently (as if in a separate buffer). From a technical perspective, it might even be considered as a globally available 'small' buffer with a history.

This would hopefully relieve some of the issues users are having with an inability to execute sections of the code independently (e.g. #1808). We could have a function which would copy the contents of a selection to the REPL and execute it there.