[DropZone] Add prop to DropZone so children control its height

[WillsonSmith]

WHY are these changes introduced?

Right now, DropZone has 4 sizes: small, medium, large, and extraLarge. These are calculated based on the width of the DropZone. These sizes determine the min-height of the component. This means you cannot have a DropZone that is narrower than the set height at a given size.

We have a case where we need the height to be narrower than what is automatically set by this component.

I've opened an issue about making this the default behaviour.

WHAT is this pull request doing?

This PR adds a prop variableHeight that skips the size calculation and makes the DropZone's height dependent on its children instead.

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

View the new "Drop zone with variable size" story in Storybook.

or

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

import React from 'react';

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

export function Playground() {
  function handleDrop() {}
  return (
    <Page title="Playground">
      <DropZone onDrop={handleDrop} variableHeight>
        <div
          style={{
            display: 'flex',
            justifyContent: 'center',
            alignItems: 'center',
            width: '100%',
            height: '100%',
          }}
        >
          <Stack vertical>
            <Button>Add file</Button>
            <span>Drop to upload</span>
          </Stack>
        </div>
      </DropZone>
    </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, pinged one of @ HYPD, @ mirualves, @ sarahill, or @ ry5n to update the Polaris UI kit

[github-actions]

🟢 No significant changes to src/**/*.tsx were detected.

[WillsonSmith]

cc/ @CarlGranstrom @sjreive @rdott @pt8o

[alex-page]

Is there a quick test we can add for this? Looking good Willson!

[WillsonSmith]

Added test 👍

[kyledurand]

👏 Good call Willson, thanks! 🥇