Retain bullet style after editing a page with lists in content editor

What does this MR do?

When you edit a page in the content editor, your original style of bullets in bullet lists are lost and are replaced with * list item style bullets.

This MR creates a way to read the markdown source of the rendered HTML to retain original markdown style for bullet lists when serializing the content back to markdown.

How to setup and validate locally (strongly suggested)

1. Create a new Wiki page.

2. Type the below code:

   * List style 1
   * List style 1
     - Sub list
   
   
   + List style 2
   + List style 2
     * Sub list
   
   
   - List style 3
   - List style 3
     + Sub list

3. Click "Use new editor".

4. Click "Save changes".

5. Edit the page again. You will note that the list bullet styles are now preserved after editing in content editor.

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.