Add a "restriction" field to the compat-data schema

[a2sheppy]

This adds an optional "restriction" field to the version
information so you can indicate that a feature is only
available on specific build channels, such as nightly or
beta releases.

See this Discourse thread in which this concept is discussed
and this proposal reached.

Includes updated schema test data and documentation.

[jpmedley]

Once in a blue moon, Chrome has a feature that will stay on nightly or stay on beta through several cycles. For example, it might be in Chrome 72 Canara, then Chrome 73 Canary, then Chrome 74 Canary. The usual process is, for example, like Chrome 72 Canary>Chrome 72 Beta>Chrome 72. (Fortunately, this only happens once in a great long while.)

Is this what you're referring to?

If so, I'd like to suggest some kind of link or popup that explains this. The behavior is odd enough that I've had a hard time explaining it to people within Chrome. (There should probably be some kind of general help link for the compat tables to explain to wiki users other things that might be vague or confusing. The meaning of 'Yes' as support value comes to mind.)

[Elchi3]

I think this PR tries to address #338.
Experimental browsers like Nightly or Canary are not our main audience (I agree with #338 (comment)).
I will try to think about this, but there are other things with higher priority.

[a2sheppy]

Okay, we can't have it both ways though. We document the version in which something was introduced, even if it isn't the release version, so we can't ignore nightly releases. We already list stuff in BCD with nightly version numbers (at least for Firefox, anyway).

[jpmedley]

BTW, my statement on Nov 26 was simply to explain one of our edge cases. I was not expressing support for the idea. I did intend to say that if I were outvoted, I would want the data caveated. From Chrome's pov the version in which something was introduced is the version in which it is enabled by default. Because this is Google's position, not mine, I don't contribute build channel or flag information except in rare cases for reasons that are decided above my pay grade.

[Elchi3]

We're not at a stage where we've agreed on a way forward. See #338 for the next steps on research here. I'm closing here.