style(skeleton): fixed placement inside of a wrapper

[sstrubberg]

Closes #9511

Added position'ing style to skeleton mixin.

Testing / Reviewing

Checkout this codepen to see what the current behavior is.

To test locally, add a textAlign: 'center' to the outermost div on the default story in the Skeleton Placeholder story.

Once you've added that, remove the styles to see the bug return.

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