Added MarkdownEditor story to testing

[green6erry]

Parent issue: https://github.com/github/primer/issues/1488

Add storybook stories for MarkdownEditor

Checklist

• Ensure your chosen component has a component directory in the src directory
• Ensure the component has a root story file in the component directory
• Ensure that the root story file exports a Default story
• (Optional) Create a Playground story
• Move all other stories for the component into a .features.stories.tsx file
• Opt the component into storybook tests
• (Optional) Create visual regression tests for the component

[changeset-bot]

⚠️ No Changeset found

Latest commit: 4b92de8

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

size-limit report 📦

Path	Size
dist/browser.esm.js	100.7 KB (0%)
dist/browser.umd.js	101.29 KB (0%)

[lesliecdubs]

Has this one been waiting for a review for awhile @green6erry? I can escalate if needed.

[broccolinisoup]

Just left a comment, let me know what you think but overall looks great!

Thanks so much for pushing this PR 🙏🏻

[broccolinisoup]

Just left a comment to remove the decorators - let me knowif you have any questions about that 😊

[broccolinisoup]

🚀