TransformValue.is2D should be an IDL attribute, not a method

For [consistency with `DOMMatrix`](https://drafts.fxtf.org/geometry/#dom-dommatrixreadonly-is2d) I think the `is2D` method on `TransformValue` should be an IDL attribute instead.  Also I'm not sure if it's worth having a separate `is2DComponent` method on `TransformComponent`. How about we just make this an `is2D` attribute too?
