Variable casting and Temporal support

[rkh]

Implements #634

Variable casting

Went for a slightly different API than in the issue.

bundle = new FluentBundle("en-US")

// these aren't needed, as they come built-in, but to illustrate the new API
bundle.addCast("number", FluentNumber)
bundle.addCast(Date, (value: Date) => new FluentDateTime(value.getTime()));

Temporal support

I've put this in a separate package for now, as Temporal is still considered experimental.

It also currently only supports Instant and ZonedDateTime.

Best illustration is probably bundle_test.js.

[rkh]

Put myself in here. You can take me out, though. I'm happy to "gift" this little library to Mozilla.

[rkh]

I am happy to work on the remaining Temporal classes, especially Duration.

We are working on a production app (pre-alpha atm) where we use a Temporal polyfill and Fluent, and we're expecting to use almost all Temporal classes eventually, so I'm also happy to keep an eye on any bugs etc that pop up in the future.

[eemeli]

I think this level of generalisation is a bit too much, and we should just add Temporal support as in #636.

If there is a real need for something like this later on, it would be best to package the functionality within FluentNumber, FluentDateTime, and related classes.