RFC Choice: expand Choice token normalization + remove str requirement

[AndreasBackx]

When looking into #2210, I wasn't sure it was a good idea to show an enum's name to the user. For example:

class MyEnum(enum.Enum):
	ONE = 1
	TWO = 2
	THREE = 3
	ALSO_ONE = 1

Would result the following if an invalid value is shown:

Error: Invalid value for '{ONE|TWO|THREE|ALSO_ONE}': 'meh' is not one of ONE, TWO, THREE.

Though it's not uncommon practice to have kebab-case, imo it's the most convenient typing for the CLI. Then you'd ideally want the user to type also-one instead of ALSO_ONE or also_one.

I realised that we have token normalization since 2.0. I feel like this is not very obvious to users as we don't make the normalization transparent to the user.

@click.option(
	"--method",
	type=click.Choice(
		["SCREAMING_SNAKE_CASE", "snake_case", "PascalCase", "kebab-case"]
	)
)

Would in the background casefold the values to screaming-snake-case, snake-case, pascalcase, and kebab-case. Though still show the following message:

Error: Invalid value for '{SCREAMING_SNAKE_CASE,snake_case,pascalcase,kebab-case}':
'meh' is not one of SCREAMING_SNAKE_CASE, snake_case, pascalcase, kebab-case.

This makes token normalization expand to cover normalization output to the user so it would now be:

Error: Invalid value for '{screaming_snake_case,snake_case,pascalcase,kebab-case}':
'meh' is not one of screaming_snake_case, snake_case, pascalcase, kebab-case.

Note that screaming_snake_case and snake_case retain their underscores because we're casefolding, not converting to kebab-case. I think we should in the future make kebab-case the default, and/or allow extending the normalization behaviour more easily so this could be converted into snake-case.

---

In addition to this normalization, I'm introducing generics into Choice and removing the requirement to have choices be str. A funny side-effect is that enums can be supported more easily. Right now you'd have to use click.Choice(list(MyEnum)) though that would result in enum aliases not showing up. We could allow one to do click.Choice(MyEnum) and handle the aliasing for them. 🤔 We could prevent people from incorrectly using click.Choice(list(MyEnum)) by failing if we detect a list of Enum types.

Let me know what you think!

Resolves #605.