Optional serde support.

[nical]

Let's add serde support behind a "serialization" feature flag (off by default).

[halzy]

Is the intent here to serialize all types including the generated ones? I doubt this issue is for disabling serde (which we no longer use) for parsing the json.

@halzy I think the intent here is to support serialization of Variants (directly), similar to how the Godot json() works, but with serde.