Outline offset doesn't support string values

[richbachman]

Description

While working with rainbow-sprinkles, I noticed outlineOffset is set to a number. That doesn't seem to render in browsers. It looks like the offset prop should take a standard length value. See the MDN docs here: https://developer.mozilla.org/en-US/docs/Web/CSS/outline-offset

Expected Behaviour

outlineOffset renders the properly set offset.

Actual Behaviour

outlineOffset value is ignored.

Affected Version

0.16.0

Steps to Reproduce

1. Add outlineOffset: true to the dynamic properties.
2. Try to use a px or rem value within Box.

Checklist

• I have read the contributing guidelines
• I have verified this does not duplicate an existing issue