Content Editor: Support audio and video in client-side deserializer

What does this MR do and why?

NOTICE: This MR is behind the preserve_unchanged_markdown feature flag.

It implements support for audio and video elements in the Content Editor’s client-side deserializer.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable preserve_unchanged_markdown feature flag.
2. Create or edit a Wiki page.
3. In the Classic Markdown Editor, paste the following Markdown snippet

   ![audio](https://gitlab.com/gitlab-org/gitlab/uploads/258801ea190283aae48c522b1710894d/audio.wav)
   
   ![video](https://gitlab.com/gitlab-org/gitlab/uploads/959a13dea972eb3a5a074b9fd1426d03/movie.mov)

   Notice that the media asset URLs point to the audio and video uploaded on this Merge Request. The client-side deserializer can’t display locally uploaded audio/video yet. We will fulfill this requirement in #359029 (closed)
4. Switch to the Content Editor. You should see the audio and video elements for the media assets.

Audio and video assets

• audio

• movie

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 #359012 (closed)