feat: add castable-video

.gitignore

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+/node_modules/

README.md

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+# `is="castable-video"`
+
+Cast your video element to the big screen with ease!
+
+The lightweight `CastableVideo` class extends the native `HTMLVideoElement` API and adds casting functionality to any video element. The API was designed to have the feel of a native browser API like the other screen presentation API's.
+
+This is the simplest way to add a Chromecast support to [Media Chrome](https://github.com/muxinc/media-chrome).
+
+
+```html
+<script src="https://www.gstatic.com/cv/js/sender/v1/cast_sender.js?loadCastFramework=1"></script>
+<script type="module" src="./castable-video.js"></script>
+
+<video
+  slot="media"
+  is="castable-video"
+  poster="https://image.mux.com/DS00Spx1CV902MCtPj5WknGlR102V5HFkDe/thumbnail.jpg"
+  src="https://stream.mux.com/DS00Spx1CV902MCtPj5WknGlR102V5HFkDe/high.mp4"
+  muted
+  crossorigin
+></video>
+```
+
+## API
+
+### Methods
+
+- `video.requestCast(options)`
+- `CastableVideo.exitCast()`
+
+### Properties
+
+- `CastableVideo.castElement`
+- `CastableVideo.castAvailable`
+
+### Events
+
+- `entercast`
+- `leavecast`
+- `castchange`
+
+e.g. `video.addEventListener('entercast', () => {})`
+
+### Attributes
+
+- `stream-type`: add `<video "stream-type="live">` for live streams.
+- `cast-src`: if Chromecast requires a different source than the one loaded.