Next Gen UI demo (#1778)

First pass at a Next Generation UI demo app. The UI needs work, feedback
gratefully accepted.

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

next_gen_ui_demo/pubspec.yaml

@@ -0,0 +1,46 @@
+name: next_gen_ui_demo
+description: Building a Next Generation UI in Flutter
+publish_to: 'none'
+version: 0.1.0
+
+environment:
+  sdk: '>=3.0.0-417.4.beta <4.0.0'
+
+dependencies:
+  cupertino_icons: ^1.0.5
+  extra_alignments: ^1.0.0+1
+  flextras: ^1.0.0
+  flutter:
+    sdk: flutter
+  flutter_animate: ^4.1.1+1
+  focusable_control_builder: ^1.0.1+1
+  gap: ^3.0.0
+  particle_field: ^1.0.0
+  provider: ^6.0.5
+  rnd: ^0.2.0
+  vector_math: ^2.1.4
+  window_size:
+    git:
+      url: https://github.com/google/flutter-desktop-embedding.git
+      path: plugins/window_size
+
+dev_dependencies:
+  analysis_defaults:
+    path: ../analysis_defaults
+  flutter_test:
+    sdk: flutter
+  flutter_lints: ^2.0.0
+
+flutter:
+  uses-material-design: true
+  fonts:
+    - family: Exo
+      fonts:
+        - asset: assets/fonts/Exo-Bold.ttf
+        - asset: assets/fonts/Exo-Medium.ttf
+  assets:
+    - assets/images/
+    - assets/fonts/
+  shaders:
+    - assets/shaders/orb_shader.frag
+    - assets/shaders/ui_glitch.frag