support REPL input

[amunger]

We need to be able to handle when user types input() and be able to wait for user to type their Python command.

[anthonykim1]

microsoft/vscode#212427 may be necessary for this need to happen.
We also discussed how Jupyter extension handles input() via attaching cell below where input() is ran, but had some visibility complains. I'm still go see how we would be able to handle and signal to wait assuming that we would eventually have access to user text input box.

Perhaps we could even call it inside execute-handler as well since input() itself would not add cell output? - for the purposes of MVP
Edit: This is not possible since it DOES append cell output of EOF when reading line when we execute input()

  File "/Users/anthonykim/Desktop/vscode-python/python_files/python_server.py", line 56, in exec_user_input
    retval = callable(user_input, user_globals)
  File "<string>", line 1, in <module>
EOFError: EOF when reading a line

[amunger]

Jupyter handles this with an input opened at the top of the window (where quick picks show up), not an extra cell.

[anthonykim1]

Input is supported. Also input with prompts from a2208ca