feat(mdxish): preserve blank line spacing in rendered output

[maximilianfalco]

🎯 What does this PR do?

See this monorepo PR: https://github.com/readmeio/readme/pull/18047

The markdown parser collapses multiple blank lines between block elements into a single paragraph break, so extra vertical spacing in the source is lost. This PR restores that spacing so the rendered output matches what the author intended.

• Add remarkRestoreBlankLines remark plugin that detects position gaps in the mdast tree where blank lines were collapsed, and re-inserts empty paragraph nodes
• Add rehypeEmptyParagraphsToBr rehype plugin that converts those empty <p> elements (invisible due to margin collapse) into <br> elements for visible spacing
• Wire both plugins into the mdxish pipeline

🧪 QA tips

• Write markdown with multiple blank lines between paragraphs, headings, lists, and code blocks — verify the extra spacing renders visibly
• Verify a single blank line (normal paragraph break) does NOT produce extra <br> elements
• Verify content inside tables, callouts, and other magic blocks is unaffected

📸 Screenshot or Loom