[Typography foundations] Refactor display variants to heading2xl-4xl

[laurkim]

WHY are these changes introduced?

Resolves #6724.

Display variants were intended to give clarity on how to apply type but ended up causing more confusion.

WHAT is this pull request doing?

Refactors the displaySm, displayMd, and displayLg variants to heading2xl, heading3xl, and heading4xl.

Storybook url.
Screenshot:

Updated Text component variants

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

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

import React from 'react';

import {Page, Stack, Text} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      {/* Add the code you want to test in here */}
      <Stack vertical>
        <Text as="h1" variant="heading4xl">
          Heading4xl variant
        </Text>
        <Text as="h2" variant="heading3xl">
          Heading3xl variant
        </Text>
        <Text as="h3" variant="heading2xl">
          Heading2xl variant
        </Text>
        <Text as="h4" variant="headingXl">
          HeadingXl variant
        </Text>
        <Text as="h5" variant="headingLg">
          HeadingLg variant
        </Text>
        <Text as="h6" variant="headingMd">
          HeadingMd variant
        </Text>
        <Text as="h6" variant="headingSm">
          HeadingSm variant
        </Text>
        <Text as="p" variant="bodyLg">
          BodyLg variant
        </Text>
        <Text as="p" variant="bodyMd">
          BodyMd variant
        </Text>
        <Text as="p" variant="bodySm">
          BodySm variant
        </Text>
      </Stack>
    </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

[github-actions]

size-limit report 📦

Path	Size
polaris-react-cjs	198.78 KB (-0.01% 🔽)
polaris-react-esm	128.35 KB (+0.01% 🔺)
polaris-react-esnext	182.99 KB (0%)
polaris-react-css	41.1 KB (-0.01% 🔽)