Improve code quality of exit_on_esc_system (#4260)

# Objective

- exit_on_esc system uses a slow and non-standard way to check for inputs

## Solution

- Use Input resource, rather than iterating events

## Context

Alternative to #4259.

crates/bevy_input/src/system.rs

@@ -1,6 +1,8 @@
+use crate::input::Input;
+use crate::keyboard::KeyCode;
 use crate::{ButtonState, KeyCode, KeyboardInput};
 use bevy_app::AppExit;
-use bevy_ecs::prelude::{EventReader, EventWriter};
+use bevy_ecs::prelude::{EventWriter, Res};
 
 /// Sends an [`AppExit`] event whenever the `ESC` key is pressed.
 ///
@@ -9,14 +11,10 @@ use bevy_ecs::prelude::{EventReader, EventWriter};
 /// This system is not added as part of the `DefaultPlugins`. You can add the [`exit_on_esc_system`]
 /// yourself if desired.
 pub fn exit_on_esc_system(
-    mut keyboard_input_events: EventReader<KeyboardInput>,
+    keyboard_input: Res<Input<KeyCode>>,
     mut app_exit_events: EventWriter<AppExit>,
 ) {
-    for event in keyboard_input_events.iter() {
-        if let Some(key_code) = event.key_code {
-            if event.state == ButtonState::Pressed && key_code == KeyCode::Escape {
-                app_exit_events.send_default();
-            }
-        }
-    }
+    if keyboard_input.just_pressed(KeyCode::Escape) {
+        app_exit_events.send_default();
+   }
 }