Ignore self-closing special tags for markdown-it

[ang-zeyu]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• Others, please explain:

Follow up to #1101
Surfaced in #1403

Overview of changes:

• Add negative lookahead to starting pattern for special tag patch for markdown-it

Anything you'd like to highlight / discuss:
This was not apparent before as the only special tag in our tests is <puml>.
These are processed before markdown-it ever touches them, resulting in false positives.

Testing instructions:

Tested on https://regexr.com/, then #1403

Proposed commit message: (wrap lines at 72 characters)
Ignore self-closing special tags for markdown-it

The markdown-it patch for special tags does not blacklist self-closed
special tags from the starting regex pattern. Thus, the starting regex
pattern incorrectly matches such a self-closing special tag, and the
ending regex pattern incorrectly seeks for a corresponding closing
special tag until EOF.

This is not apparent in the functional tests which have such tags
(<puml>), because puml tags are processed before markdown.

Let's add a negative lookahead to the starting pattern to blacklist
such tags.

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features (architectural restructure in V3 baseline #1403 surfaces this with our existing tests)
• Linked all related issues
• No blatantly unrelated changes
• Pinged someone for a review!