[Link, Plain button] Add underline to Links and Plain button on hover

[mirualves]

WHY are these changes introduced?

Fixes https://github.com/Shopify/polaris-ux/issues/147

The current hover/focus styles rely on a change in color alone which isn't visible in Windows High Contrast, and may be difficult for other low vision users to distinguish. An underline that's displayed when a user starts to interact with the link would help cement the idea that it's a link and clarify exactly what that particular link does.

WHAT is this pull request doing?

Add underline to Links and Plain button on hover, so it doesn't rely on color alone for accessibility.

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,
  Button,
  Link,
  Stack,
  Card,
  FooterHelp,
  TextContainer,
} from '../src';

interface State {}

export default class Playground extends React.Component<{}, State> {
  render() {
    return (
      <Page title="Playground">
        <Card>
          <Card.Section>
            <TextContainer>
              This is some text and <Link url="#">this thing right here</Link>{' '}
              is a link.
            </TextContainer>
          </Card.Section>
          <Card.Section>
            <Stack>
              <Button plain>View shipping settings</Button>
              <Link url="https://help.shopify.com/manual">
                fulfilling orders
              </Link>
              <Link url="https://help.shopify.com/manual" external>
                Shopify Help Center
              </Link>
            </Stack>
          </Card.Section>
        </Card>
        <FooterHelp>
          Learn more about{' '}
          <Link url="https://help.shopify.com/manual/orders/fulfill-orders">
            fulfilling orders
          </Link>
          .
        </FooterHelp>
      </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]

Adding a note that this is a partial fix. It does help users find the links on interaction, but users still might have challenges finding the links in the first place. Will take a closer look in a bit!

[dpersing]

🎉 Thanks so much for your patience, @mirualves !