Type declaration should allow `effect={null}` for svelte

[Khoulaiz]

The current type declaration only allows the four effects for the attribute.

In Svelte you set an attribute to a null value if you don't want to set the attribute at all.

That works without any problems right now, but the above type declaration gives me an error from the linter.

Example code that works but creates a false error in eslint:

 <SkeletonBlock effect={$isMotionReduce ? null : "wave"} width="84px" height="84px" borderRadius="" />