Fix: Allow self-closing tags in Markdown again

[hippotastic]

Changes

• After our switch to the MDX-like parsing logic in Beta 33 (PR Improve Markdown + Components usage #3410), Markdown parsing has become a lot more strict. This caused commonly used syntax in Markdown to become invalid.
• This PR fixes one of the most common issues concerning self-closing tags like <br>. They are now considered valid again by wrapping the MDX JSX tag parsing logic.
• Fixes 🐛 BUG: Expected a closing tag when markdown renders HTML elements that do not need closing tags #3458.

Testing

• Added a new test case to @astrojs/markdown-remark.
• Ran all tests locally.
• Performed a full build of Astro Docs without issues.

Docs

• Not a visible change, just a bugfix.

[changeset-bot]

🦋 Changeset detected

Latest commit: 511b070

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@astrojs/markdown-remark	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[natemoo-re]

Wow, this is an incredibly awesome fix! Thank you! ❤️