Add exit_on_esc_system to examples with window (#2121)

This covers issue #2110

It adds the line `.add_system(bevy::input::system::exit_on_esc_system.system())` before `.run()`
to every example that uses a window, so users have a quick way to close the examples.

I used the full name `bevy::input::system::exit_on_esc_system`, I thought it gave clarity about being a built-in system.

The examples excluded from the change are the ones in the android, ios, wasm folders, the headless 
examples and the ecs/system_sets example because it closes itself.

Author: giusdp

examples/game/alien_cake_addict.rs

@@ -40,6 +40,7 @@ fn main() {
                 .with_run_criteria(FixedTimestep::step(5.0))
                 .with_system(spawn_bonus.system()),
         )
+        .add_system(bevy::input::system::exit_on_esc_system.system())
         .run();
 }
 

examples/game/breakout.rs

@@ -21,6 +21,7 @@ fn main() {
                 .with_system(ball_movement_system.system()),
         )
         .add_system(scoreboard_system.system())
+        .add_system(bevy::input::system::exit_on_esc_system.system())
         .run();
 }