Redundant customElements definition for Web Component 'turbo-cable-stream-source' causes error in some clients

[riffdr]

This

customElements.define("turbo-cable-stream-source", TurboCableStreamSourceElement);

Should be wrapped within an undefined check like the rest of the turbo custom elements. It randomly throws errors on the client that are being reported by Sentry from our Rails application. Can someone take care of this, or may we open a pull request?

if (customElements.get("turbo-frame") === undefined) {
  customElements.define("turbo-frame", FrameElement);
}

if (customElements.get("turbo-stream") === undefined) {
  customElements.define("turbo-stream", StreamElement);
}

if (customElements.get("turbo-stream-source") === undefined) {
  customElements.define("turbo-stream-source", StreamSourceElement);
}

[dhh]

cc @seanpdoyle

[dhh]

Fixed via #410.