Add token for slim border radius

[kyledurand]

WHY are these changes introduced?

This token could be useful here

So I think it's worth adding and adding it now means we can mitigate token work in future work

WHAT is this pull request doing?

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
• For visual design changes, ping @ sarahill to update the Polaris UI kit

[sarahill]

🚢

[github-actions]

size-limit report

Path	Size
cjs	166.06 KB (+0.01% 🔺)
esm	96.6 KB (+0.01% 🔺)
esnext	143.47 KB (+0.01% 🔺)
css	34.53 KB (0%)