Add CustardUI script as bundled custom plugin

[gerteck]

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:

Overview of changes:

Adds CustardUI as a plugin

Closes #2901

Anything you'd like to highlight/discuss:

Testing instructions:

Proposed commit message: (wrap lines at 72 characters)

Add CustardUI script as bundled custom plugin

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).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.32%. Comparing base (dcff376) to head (bd7db70).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2906   +/-   ##
=======================================
  Coverage   71.32%   71.32%           
=======================================
  Files         133      133           
  Lines        7234     7234           
  Branches     1682     1713   +31     
=======================================
  Hits         5160     5160           
+ Misses       2068     1973   -95     
- Partials        6      101   +95     

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