Cannot add custom key codes in 3.0

It looks like 3.0 changed the Keys object to an Enum subclass. This makes it impossible to add new key sequences to it dynamically, as the Enum values are determined at class creation time. I have been doing this as described in https://github.com/prompt-toolkit/python-prompt-toolkit/issues/892.

I don't know if there's an easy way to enable this while keeping it an Enum. I would suggest reverting that change. Having an official API to add custom key sequences would be nice as well, although doing it manually isn't a huge deal right now.

As per https://github.com/prompt-toolkit/python-prompt-toolkit/issues/892, it would obviously be good to have some of the more esoteric codes built in so that you don't have to do this. But I think even with fuller coverage the ability to add custom codes is something that should be supported.

Start by reviewing the 3.0 change that made the Keys object an Enum subclass and the custom key-sequence approach described in issue #892. Done means users can officially add custom key codes or sequences without relying on manual changes, with behavior covering the requested esoteric codes.