Add Commonmark support to Content Editor

What does this MR do?

Adds Commonmark support to the Content Editor component. In other words, it allows displaying and editing the types of content described in the Commonmark specification:.

There are some caveats that I mention in the following table. They are related to the way the Content Editor generates Markdown like the bullet-list style. We don’t need to address formatting preferences in the Editor MVC and that’s the reason I avoid further customization in this MR.

Content Type	Notes
Paragraph	✅
Heading	✅
ThematicBreak	✅
Blockquote	✅
BulletList	✅ CAVEAT: Does not remember preferred bullet: * or - yet. I think this requirement shouldn’t be part of the MVC
OrderedList	✅ CAVEAT: Does not remember whether to only use 1.s or 1., 2., 3. yet. I think this requirement shouldn’t be part of the MVC
ListItem	✅
Link definitions and references	✅ CAVEAT: the editor will convert these into normal links. We need some backend support to output reference definitions when they are used in existing Markdown. I think this requirement shouldn’t be part of the MVC
Link	✅
Image	✅
Code block	✅
Text	✅
Emphasis	✅
Strong	✅
InlineCode	✅

Screenshots (strongly suggested)

This MR does not introduce user-facing changes.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because it does not introduce user-facing changes.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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 #325735 (closed)