Add stylelint-polaris/coverage rule

[aaronccasanova]

WHAT is this pull request doing?

This PR introduces a stylelint-polaris/coverage rule for categorizing and applying other built-in or custom rules.

The rule simply accepts an object where keys denote categories and values are a standard Stylelint rules config object:

// .stylelintrc
{
  "plugins": ["@shopify/stylelint-polaris/plugins/coverage"],
  "rules": {
-   "at-rule-disallowed-list": ["keyframes"],
-   "color-named": "never",
-   "color-no-hex": true
    "stylelint-polaris/coverage": {
      "motion": {
+        "at-rule-disallowed-list": ["keyframes"]
      },
      "colors": {
+        "color-named": "never",
+        "color-no-hex": true
      },
    }
}

Note: Error messages from built-in or custom rules are intercepted and reformatted to display the failing coverage category e.g.

- "Unexpected at-rule "keyframes" (at-rule-disallowed-list)"
+ "Unexpected at-rule "keyframes" (stylelint-polaris/coverage/motion)"

By keeping the existing rules and reorganizing them into categories:

• developers are provided more meaningful error messages, and
• we have ability to leverage the categorized failure data on the coverage site

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

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

import React from 'react';
import {Page} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      {/* Add the code you want to test in 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

[chloerice]

💥 🚀

[lgriffee]

💯🚢✨🎉