Prevent unintended keyboard input during focus.

[kosayoda]

winit generates is_synthetic: true KeyboardInput events whenever a window gains or loses focus, which is mistakenly picked up as a intentional input event.

Fixes #2621.

Prior Art

• Fixed in egui since Ignore synthetic key presses emilk/egui#4514
• Fixed in bevy since Phantom key presses on window focus change (alt+tab, etc) bevyengine/bevy#13299
• Fixed in floem since fix: ignore synthetic keyboard input lapce/floem#387
• Discussion about synthetic key events as a whole in winit

[hecrj]

Great! Thought this was going to need us to juggle focus state.

Thank you.