Add messaging APIs and their infra

[autonome]

Everything in this area is grouped in the Communications section of the HTML spec. I added a group called Messaging instead of going the spec route, since it aligns more with how developers refer to these features.

I joined MessageChannel and MessagePort into Channel Messaging, aligning with how the spec, MDN and Caniuse do it.

MessageEvent is implemented in numerous places, but doesn't seem like a standalone feature... but messageerror was already done, so I followed that precedent here, adding the various implementation keys as well.

A couple of notes on postMessage:

• Name: Caniuse calls it Cross-document messaging - which is how the spec describes it too. I dug around and few references by developers to Cross-document messaging, but a zillion to postMessage, so I'm erring on the side of usage-in-the-wild. Sometimes tutorials just called it Web Messaging also, but they pre-dated all the newer messaging stuff.
• Composition: I pulled in the implementations and put them in a feature together. There's some trade-off against discoverability of the API in the implementation itself. Really a limitation of the "one key, one home" approach in situations like this.

This whole feature group has this same challenge: Do you put the key in the implementation (eg, putting postMessage in each of Window, ServiceWorker, etc) or put the implementation keys in the feature. Might be worth a deeper discussion with the group. I'll bring this up at the meeting this week.

Features previously done in this area:

• BroadcastChannel
• messageerror

Features added here:

• Channel Messaging - includes MessageChannel and MessagePort
• postMessage
• EventSource
• MessageEvent

Group added here:

• Messaging

[ddbeck]

Very close now. Two small fixes to make and refreshing the dist, then we can land this. Thank you!