[Feature] Add number pair form field
Add an editor for a number pair that can be used in FAST tooling form.

This issue is now redundant with the upcoming removal of the CSS editor and the addition of the CSS property control. See microsoft/fast#3780.
@janechu These sound like different things. The goal of the design surfaces was to function more like traditional design software than devtools. A general css editor should be paired with more declarative options so we can call out the common attributes you'd want to change, or attributes that are not defined in css. I hope we're not losing the declarative controls to the generic.

Hmm, I'll re-open, I think I closed this because I thought it had to do with previous work on the CSS property react component that was removed in v2. We need to start with generic first and then create specific controls for the above use cases.