Add function/fn/globe key to `Modifiers`

**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 🌐︎.

No source files or tests are named in the issue. Start by locating the Rust definition of `Modifiers` and the platform keyboard-input mapping, then inspect existing modifier tests or examples. Done means the function/fn/globe key is exposed as a modifier and the relevant input behavior is covered by passing tests.

Written by the indexing model from the issue text.