Support Variant type from as-variant

[mattjohnsonpint]

The as-variant library is a dependency:

as-json/package.json

Lines 46 to 50 in 4915854

	"dependencies": {
	"as-string-sink": "^0.5.3",
	"as-variant": "^0.4.1",
	"as-virtual": "^0.1.9"
	},

But it's not used in any code. An attempt to serialize a variant:

const v = Variant.from(123);
const s = JSON.stringify(v);

fails with: Could not serialize data of type Variant. Make sure to add the correct decorators to classes.

Which of course, we can't do because it's an external library.

@JairusSW - Given your comment in #49 (comment), can you please look in to adding support for variants so they work?

Thanks.

[JairusSW]

Yeah, that'll be coming in the next big release.
I'll start pushing out my local changes to the devlop branch