(split from #977)

I really want to avoid a situation where someone writes a message using some optional extensions, and the message works in ICU4C, and then when they go use an implementation that doesn't implement the same optional extensions (like ICU4X), their message doesn't work. Avoiding this kind of situation is, in my mind, the only job of a specification.

It would make me slightly happier if implementations with optional extensions required the client code to "opt in" to using them. For example, something like

let mf = new Intl.MessageFormat("de", { extensions: ["calendarOption"] })

And then messages with a :datetime calendar="..." option will work only if the stated extension is enabled. If someone tries building an ICU4X formatter with that extension, we'll know ahead of time in the constructor that the extension is not available, instead of waiting to see the message payloads.