Add Element.animate and Animation

[karlseguin]

These are dummy implementations, but they do expose the ready and finished promise, and do resolve the finished promise, so it should unblock basic cases.

[mookums]

looks fine to me. just curious what our general consensus is on the file-as-a-struct thing?

[karlseguin]

I personally find it troublesome. Always seems like sooner or later, you'll want to convert the type to a generic, or a union. But I keep trying to use it, because I find const SomeLongName = @import("some_long_name.zig").SomeLongName insulting to the aesthetics of the code.

My logic has been that, for these WebAPIs at least, they should be razor-focused. There's no reason that Animation or DataSet should be anything but a POJO..and the file-as-a-struct thing helps make the point.

You?