fix(jsx-key): handle shorthand fragment syntax

[kaykayehnn]

In eslint/eslint#9664 eslint added support for the shorthand fragment syntax (<></>) as a new node type: JSXFragment. The jsx-key rule hasn't been updated to handle it correctly so in some cases where it should report errors it doesn't. Here is an example

[1, 2, 3].map(x => <>{x}</>);

This PR fixes this and adds some tests. The error messages could probably be improved and maybe we should add an example to the docs?

[ljharb]

I can't decide if this is a breaking change or not; if so, it'd need to go behind an option.

[kaykayehnn]

Yeah, this would be a breaking change. I added it behind a checkFragmentShorthand option.
It would make sense that by default this option should be enabled. Maybe the option could be removed and be the default in the next major?

[yannickcr]

Merged, thank you 🙂

(and yes it will be definitely set as the default in next major)

[kaykayehnn]

Great! I was planning to update the docs and tweak the message a bit, I should have left a comment about that. Is it ok if I make a follow up PR for that?