feat(Step): add unstackable prop to Step.Group

[layershifter]

Ref #1851.
Ref #3714.

This PR adds unstackable prop to the Step.Group component.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2b906ed). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1856   +/-   ##
=========================================
  Coverage          ?   99.67%           
=========================================
  Files             ?      145           
  Lines             ?     2474           
  Branches          ?        0           
=========================================
  Hits              ?     2466           
  Misses            ?        8           
  Partials          ?        0

Impacted Files	Coverage Δ
src/elements/Step/StepGroup.js	100% <ø> (ø)

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[levithomason]

Released in [email protected].