Support for multiple-choice

[asfaltboy]

Requirements

I want to implement a multiple choice prompt, where a user is given the option to input more than one choice. Here a list of some basic requirements:

1. validation function should be called once for each choice, but mutually exclusive validation (using all values entered so far) should also be possible.
2. each response should be registered visually (highlighted or ✔️ (checked) in the choice table/list)
3. result type must be a list of values (e.g list[str])

My workaround

I hacked together this quick thing to allow comma separated answers to be validated, it solves some of the issues above, but it's clearly not the best UX though, especially in regard to selecting and registering choices.

class MultipleChoicePrompt(PromptBase[list[str]]):
    """Support returning multiple choices"""

    response_type = list[str]

    def process_response(self, value: str) -> list[str]:
        result = []
        value = value.strip()
        for sub_value in value.split(","):
            sub_value = sub_value.strip()
            if not self.check_choice(sub_value):
                raise InvalidResponse(self.illegal_choice_message)
            result.append(sub_value)
        if not result:
            raise InvalidResponse(self.illegal_choice_message)
        if len(result) > 1:
            if {"not_applicable", "na"} & set(result):
                raise InvalidResponse(
                    f"[prompt.invalid.choice]Cannot combine results: {result}"
                )
        return result