Add new sample simple_sdf (#2835)

Add a new sample, `simple_sdf`. This is similar to `simple_shader`, but
it focuses on using Signed Distance Functions within the fragment
shader.

Relates to issue
[#183043](https://github.com/flutter/flutter/issues/183043)

## 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].
- [ ] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).


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

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

simple_sdf/pubspec.yaml

@@ -0,0 +1,23 @@
+name: simple_sdf
+description: Using a shader, simply.
+publish_to: 'none'
+version: 1.0.0+1
+resolution: workspace
+
+environment:
+  sdk: ^3.9.0-0
+
+dependencies:
+  flutter:
+    sdk: flutter
+  flutter_shaders: ^0.1.0
+
+dev_dependencies:
+  analysis_defaults:
+    path: ../analysis_defaults
+  flutter_test:
+    sdk: flutter
+flutter:
+  uses-material-design: true
+  shaders:
+    - shaders/SDF.frag