[Tabs - Disclosure] Add rollover on Disclosure and high contrast mode

[mirualves]

WHY are these changes introduced?

Resolves: https://github.com/Shopify/polaris-ux/issues/259

WHAT is this pull request doing?

This pull requesting is doing two things:

• Adding a rollover state to Disclosure on Tabs (the same way we have on Home Cards.
• Adding a rollover state for the same element for Windows High Contrast mode.

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

Copy-paste this code in playground/Playground.tsx:

import * as React from 'react';
import {Page} from '@shopify/polaris';

interface State {}

export default class Playground extends React.Component<never, State> {
  render() {
    return (
      <Page title="Playground">
        {/* Add the code you want to test here */}
      </Page>
    );
  }
}

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide

[dpersing]

This looks good to me. Great job fitting the styles into the existing High Contrast palettes!

[alex-page]

Nice work @mirualves