markdown: fix adjacent md syntax in blockqote

[fiji-flo]

Description

Picking up work on #9099 with a slightly other approach than #9125. It's still hacky but the best way would require way more refactoring.

I'm checking whether the restored node value matches a newline followed by > (with optional spaces if it does just return the original node.

Checklist

• [x] I’ve added tests to confirm my change works.
• [ ] (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory).
• [ ] (If the change is user-facing) I’ve added my changes to changelog_unreleased/*/XXXX.md file following changelog_unreleased/TEMPLATE.md.
• [x] I’ve read the contributing guidelines.

✨Try the playground for this PR✨