Harmonise AnimatedSprite3D and its 2D counterpart

Add the following properties to AnimatedSprite3D:
- `backwards` parameter in `play()`;
- `speed_scale`.

Both classes' internals are more similar, down to the line spacings. They've also been updated to be clearer and less inconsistent (e.g. `!frames.is_valid()` -> `frames.is_null()`, use SceneStringNames instead of CoreStringNames, rename the internal _queue_update to _queue_redraw)

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