This is so we can make the switch to the new input object seemless on encountering EOFError in previous input object.
We also detect whether we are going to go into an infinite EOFError state (i.e if Pry.config.input generates EOFError) and so we break out of the REPL with an error message
and a throw(:breakout)
input_stack allows Pry instances to seemlessly continue reading input from other input objects on the stack when the current object generates EOF
Useful when you need to temporarily divert reading to another object and then switch back to another object
