Fallback to flat when no oxr runtime is found (#51)

* basics done? now to the fun part: changing the ENTIRE lib to work with xr and non xr

* updated stuff and renamed file

* actually add the renamed file into git lol :3

* made lib fallback to flat when no runtime is found but can't compile with default settings under those circumstances

examples/demo/src/setup.rs

@@ -56,15 +56,15 @@ pub fn setup_scene(
     ));
 
     // light
-    commands.spawn(PointLightBundle {
-        point_light: PointLight {
-            intensity: 1500.0,
-            shadows_enabled: true,
-            ..default()
-        },
-        transform: Transform::from_xyz(4.0, 8.0, 4.0),
-        ..default()
-    });
+    // commands.spawn(PointLightBundle {
+    //     point_light: PointLight {
+    //         intensity: 1500.0,
+    //         shadows_enabled: true,
+    //         ..default()
+    //     },
+    //     transform: Transform::from_xyz(4.0, 8.0, 4.0),
+    //     ..default()
+    // });
     // camera
     commands.spawn((Camera3dBundle {
         transform: Transform::from_xyz(0.25, 1.25, 0.0).looking_at(