Add support for function keys

[ambihelical]

Is your feature request related to a problem? Please describe.

It would be very useful if egui had support for function keys F1, F2, etc as they are great to bind for special purposes.

Describe the solution you'd like

Support for Fn keys in Key enum and handling on at least desktop platforms. Compile time feature if necessary.

Describe alternatives you've considered

I don't know of any alternative solutions other than choosing other key bindings. However a user may expect F1 to bring up help, and might appreciate function keys for other application functions.

Additional context

[emilk]

Just go ahead and add them if you want them! Just make sure to test them on web too!

[ambihelical]

Looks like this was implemented in PR #1665, so closing.