Preserve multiline blockquotes in the Content Editor

What does this MR do and why?

CONTEXT: This change is behind the preserve_unchanged_markdown feature flag.

It preserves the multiline blockquote syntax when saving changes using the Content Editor. This is achieved by differentiating between multiline and single-line blockquotes in the Markdown parser. The Content Editor already knows how to use this metadata to generate Markdown that respects the user’s preference.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the preserve_unchanged_markdown feature flag.
2. Edit or create a Wiki page.
3. Add a multi-line blockquote in the Classic Markdown Editor.
4. Switch to Rich Text mode.
5. Modify the Content of the multi-line blockquote.
6. Switch back to the Classic Markdown Editor. You should see that the multiline blockquote syntax is used.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #359008 (closed)