Resolve "Display non-markdown content in the WYSIWYG mode of the SSE::Identifiers (inline instance)"

What does this MR do?

It prevents editing of inline identifier instance syntax in the Static Site Editor's WYSIWYG mode.

Use case examples:
- Majority: two bracket pairs, back-to-back, each with content (including spaces)
  - `[environment terraform plans][terraform]`
  - `[an issue labelled `~"master:broken"`][broken-master-issues]`
- Minority: two bracket pairs the latter being empty or only one pair with content (including spaces)
  - `[this link][]`
  - `[this link]`

In a follow-up I'll be handling the unique case where inline code in the content section is possible.

In iteration, I can envision:

• Our link button capable of handling these identifier links
• Dedicated support for a UI for selecting existing or adding new identifiers
• Rendering the actual link akin to the markdown link syntax `content

Screenshots

span solution (incomplete):

WYSIWYG (span)	Markdown (span)

a solution (complete):

WYSIWYG (a)	Markdown (a)

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