Updated SSE front matter language support

What does this MR do?

It adds support for TOML and JSON front matter language types to the static site editor.

• The main goal of this MR is to facilitate a downstream MR (MR3) where I anticipate adding an engine that has parse and stringify functions so each definition can manage the actual parsing of the respective front matter language type to a JavaScript object for further downstream use of having a dedicated UI for front matter editing
• Secondarily, SSE use outside the handbook with .md files containing TOML or JSON front matter will now properly be parsed out of WYSIWYG mode

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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

Closes #241000 (closed)