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/README.md

@@ -0,0 +1,25 @@
+# element_embedding_demo
+
+This package contains the application used to demonstrate the
+upcoming Flutter web feature: "Element Embedding".
+
+This was first shown on the Flutter Forward event in Nairobi (Kenya), by Tim
+Sneath. [See the replay here](https://www.youtube.com/watch?v=zKQYGKAe5W8&t=5799s).
+
+## Running the demo
+
+The demo is a Flutter web app, so it can be run as:
+
+```terminal
+$ flutter run -d chrome
+```
+
+## Points of Interest
+
+* Check the new JS Interop:
+  * Look at `lib/main.dart`, find the `@js.JSExport()` annotation.
+  * Find the JS code that interacts with Dart in `web/js/demo-js-interop.js`.
+* See how the Flutter web application is embedded into the page now:
+  * Find `hostElement` in `web/index.html`.
+
+_(Built by @ditman, @kevmoo and @malloc-error)_