Add default Full Screen command

### What is the problem or limitation you are having?

Much like #796, the default Full Screen shortcut(s) is(are) missing. Toga does not send the app into Full Screen when the default platform keyboard shortcut for Full Screen is pressed. (tested on macOS)

On macOS, two shortcuts are available by default: `Command + Control + F` and `Fn + F`.
On Windows and Linux, it's `F11`.

### Describe the solution you'd like

Make the default platform keyboard shortcut for Full Screen trigger Full Screen.

### Describe alternatives you've considered

None

### Additional context

_No response_

The issue does not name files or tests. Start by locating Toga's existing default keyboard-shortcut and Full Screen handling, then inspect the macOS, Windows, and Linux paths. Done means each platform's documented default Full Screen shortcut triggers Full Screen.

Written by the indexing model from the issue text.