Implement rot13 cypher

[Opisek]

Why

ROT13 cypher is a weak cypher that allows you to quickly masquerade some text. It's a self-inverse function, so applying it again will decrypt the text. Original Vim implements ROT13 cypher under the g? keybinding.

It is particularly useful for note-taking as an example. In a studying scenario, you can write down questions with answers while applying ROT13 on the latter. To check the answers, you simply need to apply the transformation again.

What changed

Added the g? operator.

Test plan

I modified the tests for gu and gU, since they implement similar functionality. I currently test:

• A few example words
• Involution of g?
• g? on a single line
• g? with motions
• g? in visual mode