Preserve unchanged markdown in content editor

What does this MR do?

Issue: #338338 (closed)

The Content Editor forgets all the formatting preferences of a Markdown file when saving changes. This is because the Content Editor re-generates the entire Markdown source from the Markdown API output instead of only generating Markdown for the parts of the file that were edited in the editor.

After this commit, the content editor will retain original markdown for all blocks that were not touched by the user. For blocks that were touched, markdown transformation still applies.

How to setup and validate locally (strongly suggested)

1. Create a new Wiki page.

2. Type the below code:

    # Heading style 1 #
   
    Heading style 2
    ---------------
   
    You could bold with **asterisks** or you could bold with __underscores__.
    You could even bold with <strong>strong</strong> or <b>b</b> html tags.
    You could add newlines in your paragraph.
   
    You could italicise with *asterisks* or you could italicise with _underscores_.
    You could even italicise with <em>em</em> or <i>i</i> html tags.
   
    As long as you don't touch a paragraph, it will ~~discard~~ <s>destroy</s>
    <del>delete</del> <strike>remove</strike> preserve the original markdown style.

3. Click "Use new editor".

4. Click save without changing anything.

5. The content editor will preserve your original markdown style for stuff you didn't change.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #338338 (closed)