feat(Modal): implement support for 3 button modals

[emyarod]

(tests will be updated when we have confirmed if the API changes are good to go)

--

Related #4607

This PR adds support for 3 button modals according to the latest modal spec. Since our existing Modal API expects only 1 primary and 1 secondary button, a secondaryButtons prop is introduced which can be used in place of the secondaryButtonText and onSecondarySubmit props. The secondaryButtons prop expects an array of up to 2 objects with the following shape:

{
  buttonText: PropTypes.string,
  onClick: PropTypes.func,
}

if an onClick handler is not supplied to the secondary buttons in a ComposedModal it will fall back to the internal handleRequestClose method by default

Changelog

New

• secondaryButtons prop
• 3 button modal styles (not including linear directional modals)

Testing / Reviewing

Confirm the 3 button modal styles appear correct and there are no visual regressions with existing supported modal styles

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[aagonzales]

The primary button is reflowing weird. It should keep a consistent top margin. Is this just a button bug unrealted to this PR?

[emyarod]

that's just directly related to the width change, since instead of 50% width they're now 25% width. if you increase the size of your viewport it would be what you'd normally expect I think

[emyarod]

modified button text vertical alignment for 3 button modals only after discussion with @aagonzales

[aagonzales]

Thanks for fixing the button text. Looks good!

[tw15egan]

Looks really great! Do you think we can add a quick blurb to the .mdx files to showcase how to use the 3 button variant of Modal and ComposedModal? The doc table doesn't seem to showcase the expected config objects / props

[tw15egan]

LGTM 👍 ✅ 🎉