Add function/fn/globe key to Modifiers

[valentinegb]

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

On macOS, the globe/fn key is used in shortcuts sometimes. For example, to enter fullscreen in almost all apps, you can press 🌐︎ F, but this is not possible currently in an egui application.

Describe the solution you'd like

The globe key is the same as the fn key, or function key, which is also common on most non-Mac keyboards. Since it's a platform-agnostic key, it makes a lot of sense for it to be a modifier in Modifiers.

Describe alternatives you've considered

Don't use fn I guess ¯\_(ツ)_/¯

Additional context

I remember reading somewhere that on macOS, applications just need to handle fn and it'll be treated as the same key as 🌐︎.