Add window::Settings::fullscreen

Corresponds to `winit:🪟:WindowAttributes::with_fullscreen`.

Currently only allows to set `Fullscreen::Borderless(None)` meaning
borderless on the current monitor, exclusive fullscreen does not
make sense for a GUI and iced does not expose monitors yet.

winit/src/conversion.rs

@@ -24,6 +24,11 @@ pub fn window_attributes(
             height: settings.size.height,
         })
         .with_maximized(settings.maximized)
+        .with_fullscreen(
+            settings
+                .fullscreen
+                .then_some(winit::window::Fullscreen::Borderless(None)),
+        )
         .with_resizable(settings.resizable)
         .with_enabled_buttons(if settings.resizable {
             winit::window::WindowButtons::all()