Permit implictly unwrapped optionals when using @SceneTree

[PadraigK]

Since I know that I'm only going to reference these properties after they are available in _ready(), I prefer to implicitly unwrap the optional. At the moment a diagnostic warning prevents me from doing this. This PR loosens the requirement a bit so that we can use ! as well as ?. It will still emit a diagnostic if the type is missing both.

class MainScene: Node {
    @SceneTree(path: "StartPosition") var startPosition: Marker2D!
    @SceneTree(path: "Camera2D") var camera: Camera2D!
}

[migueldeicaza]

Thank you for the contribution!