Return DOM fragment in Markdown Serializer

What does this MR do and why?

This is a small refactoring that changes the return value of the content_editor/services/markdown_deserializer module. This module accepts a HTML as a String value and returns a ProseMirror doc. However, in the middle of this conversion process, the HTML is converted into a DOM Fragment first.

This refactoring returns the ProseMirror document along with that DOM Fragment.

Why is this useful?

It helps us to load highlight.js asynchronously !82638 (merged)

Screenshots or screen recordings

No user-facing changes

How to set up and validate locally

Really, everything works exactly the same 😄

MR acceptance checklist

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

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