Add support for description lists in content editor

What does this MR do?

Issue: #339537 (closed)

Allow support for description lists in content editor

Content editor now supports rendering and editing dl/dd/dt elements. Creating and editing these elements works in a very similar way to regular lists.

Related MRs:

• Add support for div, figure, figcaption in content editor
• Add support for collapsible content in content editor
• Add support for description lists in content editor (⬅ This MR)

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Rendering a description list

1. Create a new wiki page, or edit an existing one

2. Type in the following HTML in the markdown editor:

   <dl>  
   <dt>HTML</dt>  
   <dd>is a markup language</dd>  
   <dt>Java</dt>  
   <dd>is a programming language and platform</dd>  
   <dt>JavaScript</dt>  
   <dd>is a scripting language</dd>  
   <dt>SQL</dt>  
   <dd>is a query language</dd>   
   </dl>  

3. Switch to new editor by clicking "use new editor".

4. The description renders successfully.

5. Save with or without making any changes.

6. Edit the page again

7. Note that the markdown / HTML for the description lists is correct.

Creating a new description list

1. Type <dl> in content editor to insert a new description list.
2. Press Enter when within a description list to create a new description term.
3. Press Tab to convert a description term to a description details.
4. Press Shift + Tab to convert a description details to a description term.
5. Press Shift + Tab again to convert it to a regular paragraph.
6. Press Enter twice on the last item to exit the description list and start a new paragraph.

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.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #339537 (closed)