Make beg_end math delimitor default

[Golddirio]

What is the purpose of this pull request?

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

Closes #2710

Overview of changes: The beg_end Math delimitor is made default without the need to configure in site.json for users. Hence, the following code is no longer needed if the user would like to use beg_end:

 {
  "plugins": [
    "mathDelimiters"
  ],
  "pluginsContext" : {
    "mathDelimiters" : {
      "delimiters": ["beg_end"]
    }
  },
}

Anything you'd like to highlight/discuss: This improvement does not affect the use of other Math delimitors. If the user would like to use other delimitors other than beg_end they still need to configure in site.json as usual.

Testing instructions:

Proposed commit message: (wrap lines at 72 characters) Make beg_end math delimitor default

---

Checklist: :ballot_box_with_check:

• [ ] Updated the documentation for feature additions and enhancements
• [x] 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).

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 52.92%. Comparing base (f681976) to head (9007072).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2737      +/-   ##
==========================================
+ Coverage   52.84%   52.92%   +0.07%     
==========================================
  Files         130      131       +1     
  Lines        7162     7173      +11     
  Branches     1591     1502      -89     
==========================================
+ Hits         3785     3796      +11     
  Misses       3072     3072              
  Partials      305      305              

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