Add Polaris migrator utilities

[aaronccasanova]

WHAT is this pull request doing?

Breaking out some key abstractions from PR #7335 so other migration folks can start using them!

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

[lgriffee]

Love these changes!!! ❤️✨🚀🎉 Thanks so much for creating a basic rem() replacement function! It's going to be super helpful for gap length replacements 🙏🏻

[sam-b-rose]

Thanks for creating these awesome utilities! Will be super useful 🔧 🧰