Fix markdown-it text centering delmiter conflict

[Incogdino]

What is the purpose of this pull request?

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

Closes #2920

Overview of changes:
The plugin tagged its delimiters with the character code of - (45) and < (60). Other double-delimiter rules (e.g. the -- "small text" rule) also derive their marker from charCodeAt, so - collided with 45 too. Since inline rules share one state.delimiters array, the center-text post-processing pass grabbed unrelated delimiters and wrongly wrapped them in <div class="text-center">.

Switched the plugin's markers to negative sentinels (-1/-2), where no real character code produces so they can't collide with any other rule.

Anything you'd like to highlight/discuss:

Testing instructions:
Added regression test combining the two rules.

npm run test in packages/core directory

Proposed commit message: (wrap lines at 72 characters)

Fix markdown-it text centering delmiter conflict

Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

---

Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

[gerteck]

Does it close #2716 as well? If so, can close that issue too!

[gerteck]

lgtm preview site fixed

[Incogdino]

@gerteck yup it fixes that. Will update the description and add some screenshots

[gerteck]

Just checked, it doesn't fix the root cause of #2716 , let's keep it open, it's a vue hydration error that is quite hard to fix

A good proxy to check is to check the 404 page... (https://deploy-preview-2921--markbind-master.netlify.app/404 )

Currently the markbind site 404 page is also broken: https://markbind.org/404

<frontmatter>
    title: Page not found
    layout: 404.md
</frontmatter>

-><p style="font-size: 10rem">404</p><-
-><p style="font-size: 1.5rem">File not found<br>Click <a href="/">here</a> to go back to the home page.</p><-

but it properly fixes #2920

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.33%. Comparing base (36426c7) to head (8b6f079).

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #2921    +/-   ##
========================================
  Coverage   71.32%   71.33%            
========================================
  Files         133      133            
  Lines        7234     7236     +2     
  Branches     1612     1714   +102     
========================================
+ Hits         5160     5162     +2     
+ Misses       2068     1973    -95     
- Partials        6      101    +95     

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[github-actions]

@Incogdino Each PR must have a SEMVER impact label, please remember to label the PR properly.

[damithc]

Thanks for the quick fix, @Incogdino 💯
Shall we do a patch release?

[Incogdino]

@damithc v7.2.1 have been released including this patch.