MWPW-193416: add MU country as a region of en_US

[3ch023]

Resolves https://jira.corp.adobe.com/browse/MWPW-193416
QA Checklist: https://wiki.corp.adobe.com/display/adobedotcom/M@S+Engineering+QA+Use+Cases

Please do the steps below before submitting your PR for a code review or QA

• C1. Cover code with Unit Tests
• C2. Add a Nala test (double check with #fishbags if nala test is needed)
• C3. Verify all Checks are green (unit tests, nala tests)
• C4. PR description contains working Test Page link where the feature can be tested
• C5: you are ready to do a demo from Test Page in PR (bonus: write a working demo script that you'll use on Thursday, you can eventually put in your PR)
• C.6 read your Jira one more time to validate that you've addressed all AC's and nothing is missing

Test URLs:

• Before: https://main--mas--adobecom.aem.live/studio.html#fragmentId=393eb336-5294-44dc-83aa-19840b8e654d&page=fragment-editor&path=sandbox
• After: https://mwpw-193211--mas--adobecom.aem.live/studio.html#fragmentId=393eb336-5294-44dc-83aa-19840b8e654d&page=fragment-editor&path=sandbox

Milo pages

• Before: https://main--cc--adobecom.aem.page/africa/products/catalog
• After: https://main--cc--adobecom.aem.page/africa/products/catalog?maslibs=mwpw-193211--mas--adobecom&clearCaches.io

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Commits

• 9222a9e ✅ (latest)
• 06a2bee ✅

[yesil]

@3ch023 #766 is already doing the same.

[afmicka]

@yesil yours is adding it for Express. This one is for Acom

[afmicka]

@3ch023 should we add this in the alphabetical order? Or you want to keep it at the bottom on purpose?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.32%. Comparing base (d313931) to head (9222a9e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #780   +/-   ##
=======================================
  Coverage   87.32%   87.32%           
=======================================
  Files         210      210           
  Lines       62870    62870           
=======================================
  Hits        54904    54904           
  Misses       7966     7966           

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