Adapt BroadcastChannel events to the new event structure

[teoli2003]

This PR updates the BroadcastChannel interface for new-style event pages. (see the template)

The onXYZ event handler pages are redirected to the eventname_event pages. As both the event pages and the onXYZ pages were existing, I used:
yarn content delete Web/API/BroadcastChannel/onmessage --redirect=/en-US/docs/Web/API/BroadcastChannel/message_event
yarn content delete Web/API/BroadcastChannel/onmessageerror --redirect=/en-US/docs/Web/API/BroadcastChannel/messageerror_event

I also corrected a few details:

• I removed the experimental banners and icons.
• I removed a gremlin (curly ").
• I fixed the reported flaws.
• I made sure to use = event => {} modern JS way of assigning events.
• I removed the uses of {{event}} macros in these pages.

BCD: mdn/browser-compat-data#14969

[github-actions]

Preview URLs

• https://pr12982.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/BroadcastChannel/BroadcastChannel
• https://pr12982.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/BroadcastChannel/close
• https://pr12982.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/BroadcastChannel/message_event
• https://pr12982.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/BroadcastChannel/messageerror_event
• https://pr12982.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/BroadcastChannel/name
• https://pr12982.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/BroadcastChannel/postMessage
• https://pr12982.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/BroadcastChannel
• https://pr12982.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/Broadcast_Channel_API

Flaws

Note! 7 documents with no flaws that don't need to be listed. 🎉

URL: /en-US/docs/Web/API/Broadcast_Channel_API
Title: Broadcast Channel API
on GitHub
Flaw count: 3

• broken_links:
  • Can't resolve /en-us/docs/Web/API/BroadcastChannel/message_event
• bad_bcd_links:
  • no explanation!
  • no explanation!

External URLs

URL: /en-US/docs/Web/API/BroadcastChannel
Title: BroadcastChannel
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/BroadcastChannel/name
Title: BroadcastChannel.name
on GitHub

No external URLs

---

URL: /en-US/docs/Web/API/BroadcastChannel/postMessage
Title: BroadcastChannel.postMessage()
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/BroadcastChannel/message_event
Title: BroadcastChannel: message event
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/BroadcastChannel/BroadcastChannel
Title: BroadcastChannel()
on GitHub

No external URLs

---

URL: /en-US/docs/Web/API/BroadcastChannel/close
Title: BroadcastChannel.close()
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/BroadcastChannel/messageerror_event
Title: BroadcastChannel: messageerror event
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/Broadcast_Channel_API
Title: Broadcast Channel API
on GitHub

No new external URLs

[Elchi3]

Should be {{InheritanceDiagram("MessageEvent")}}

[Elchi3]

{{InheritanceDiagram("MessageEvent")}}