Pass a Markdown serializer to the Content Editor

What does this MR do?

This MR Refactors the Content Editor to decouple Markdown serializing from the Editor itself. The goal is allowing to customize Markdown serialization by configuring a separate serialization object independently from the Content Editor itself.

What are the serialization/deserialization targets?

When deserializing, we want to convert a Markdown string into a ProseMirror document and serialization performs the inverse operation.

Why is this useful?

We have to parse and render Markdown before converting it into a ProseMirror document. The process of parsing and rendering Markdown varies across use cases. That’s why we want to decouple this operation from the Content Editor.

Screenshots (strongly suggested)

This MR does not introduce user-facing changes. The Content Editor is not used by any feature yet.

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 this MR 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