Add Table of Contents Content Editor Extension

What does this MR do?

This MR addresses #338270 (closed) by adding a Content Editor extension for handling GFM's Table of Contents functionality (with [[_TOC_]]).

Thanks!

Thanks so much @ealcantara @himkp & @leetickett for pairing with me on this! 🎉

Screenshots or Screencasts (strongly suggested)

Screenshot	Demo
	20210910_toc_content_editor_demo

How to setup and validate locally (strongly suggested)

1. Start GDK and log in

2. Go to a project's wiki

3. Create a new page with something like:

   [[_TOC_]]  
   
   # Lorem  
   
   ## Ipsum  
   
   ## Dolar  
   
   # Sit  

4. After saving, click "Edit" to edit the newly created page

5. In the edit page, click "Use new editor"

6. You should see a box with "Table of Contents"

7. You can try entering [TOC] or [[_TOC_]] from the new editor which should insert another "Table of Contents" block.

8. Make sure that nothing changes after save

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.

Related to #338270 (closed)