Annotate Choice.disabled as str or bool

[chanicpanic]

What is the problem that this PR addresses?

Choice.disabled is annotated as an optional str but can also be a bool:

questionary/questionary/prompts/common.py

Lines 434 to 438 in dc46610

	"{}".format(
	""
	if isinstance(choice.disabled, bool)
	else " ({})".format(choice.disabled)
	),

Partially related to #251 (comment)

How did you solve it?

This PR changes the type of Choice.disabled and related annotations to Optional[Union[str, bool]].

Checklist

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