Readonly MultiSelect

[lee-chase]

Contributes to #2177

Closes #12438

Adds the read-only feature to the MultiSelect component.

Changelog

Added

• Adds readOnly property to MultiSelect and functionality
• Adds test for read-only MultiSelect
• Adds styling for read-only MultiSelect.

Testing / Reviewing

Reviewed the result in Storybook and a added read-only test.

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[tw15egan]

LGTM 👍 ✅

[tay1orjones]

Minor nit on the story composition, but otherwise looks good to go!