feat: add custom_key_binding argument to add custom keyboard handlings

[baturayo]

What is the problem that this PR addresses?
Users were not able to customize keyboard inputs. Thanks to this PR, we can now customize the keyboard input as shown below thanks to new custom_key_binding argument. For now, custom_key_binding field has been added for select, text, checkbox, and confirm.

We will add a custom key handling for the s button in this example. Whenever user press s, we will return _skipped_ string.custom_key_binding argument takes a dictionary where the key is either a string or prompt_toolkit.keys.Keys object the value is a callable that accepts a prompt_toolkit.key_binding.KeyBinding event.

questionary.select(
    "What do you want to do?",
    choices=["Order a pizza", "Make a reservation", "Ask for opening hours"],
    instruction="(<s> to skip the question)",
    custom_key_binding= {
            "s": lambda event: event.app.exit(result="_skipped_")
        }
).ask()

or the same example can be written as

def custom_handler(event):
     return event.app.exit(result="_skipped_")

questionary.select(
    "What do you want to do?",
    choices=["Order a pizza", "Make a reservation", "Ask for opening hours"],
    instruction="(<s> to skip the question)",
    custom_key_binding= {
            "s": custom_handler
        }
).ask()

Checklist

• I have read the Contributor's Guide.
• I will check that all automated PR checks pass before the PR gets reviewed.

[kiancross]

Thanks for the PR.

I've done a quick pass of code changes and made some suggestions. I think you've disabled the option for reviewers to apply changes themselves, so can you look over these changes and then apply them? Once you've done that, I'll do a more comprehensive review.

FYI, I might have missed changing custom_key_binding to custom_key_bindings in some places - please fix if I have!

[baturayo]

Thanks for your review @kiancross, I fixed your feedbacks :)

[kiancross]

I wonder if a better interface might be

{"c": lambda: "custom"}

or even just

{"c":  "custom"}

The other code would be modified to something like this

    if custom_key_bindings is not None:
        for key, func in custom_key_bindings.items():
            bindings.add(key, eager=True)(lamba event: event.app.exit(result=func()))
            # or, if `func` is just a string
            bindings.add(key, eager=True)(lamba event: event.app.exit(result=func))

The reason for this is that I don't think we expose event.app anywhere else, and in most (all?) places, we just return a value, which is handled by internal code, rather than requiring users to call event.app.exit explicitly.

What do you think?