Publish web_embedding (#1777)

## 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


Co-authored-by: Mark Thompson
<2554588+MarkTechson@users.noreply.github.com>
Co-authored-by: David Iglesias <ditman@gmail.com>

Co-authored-by: Mark Thompson <2554588+MarkTechson@users.noreply.github.com>
Co-authored-by: David Iglesias <ditman@gmail.com>

web_embedding/element_embedding_demo/pubspec.yaml

@@ -0,0 +1,23 @@
+name: element_embedding_demo
+description: A small app to be embedded into a HTML element (see web/index.html)
+publish_to: 'none'
+version: 1.0.0+1
+
+environment:
+  sdk: '>=3.0.0-0 <4.0.0'
+
+dependencies:
+  cupertino_icons: ^1.0.2
+  flutter:
+    sdk: flutter
+  js: ^0.6.6
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+  flutter_lints: ^2.0.0
+
+flutter:
+  uses-material-design: true
+  assets:
+    - assets/dash.png