feat: add readonly dropdown

[lee-chase]

Contributes to #2177

Closes #12432

Adds the read-only feature to the Dropdown component.

Changelog

Added

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

Testing / Reviewing

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

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[alisonjoseph]

LGTM!

Looks like you have a couple merge conflicts, and one super small thing, would be nice to add

  readOnly: {
    control: {
      type: 'boolean',
    },
    defaultValue: false,
  }, 

to the Playground.argTypes in storybook

[lee-chase]

LGTM!

Looks like you have a couple merge conflicts, and one super small thing, would be nice to add

  readOnly: {
    control: {
      type: 'boolean',
    },
    defaultValue: false,
  }, 

to the Playground.argTypes in storybook

@alisonjoseph conflicts resolved

The readOnly prop appears in the playground but has no default. I'm not sure adding a default value to an optional property would be correct. If anything Storybook should allow an optional property to be reset to undefined.

[tw15egan]

LGTM pending design review 👍 ✅

[aledavila]

@thyhmdo if you could review this when you have time please !

[thyhmdo]

Looks good.