feat(isMobilePhone): add Montserrat (en-MS)

[whoalin1]

Summary

Add en-MS (Montserrat) mobile validation for isMobilePhone.

NANP NPA 664 (+1 664). Accepts Digicel 392–396 and FLOW 492–496 mobile NXX ranges (ITU / MICA numbering plan).

Refs #1761

References

• https://www.itu.int/oth/T020200008F/en (Montserrat NNP)
• Format: +1 664 NXX XXXX

Checklist

• PR contains only related changes
• README updated
• Tests written
• References provided

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9ff3424) to head (2ddb253).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2914   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2599      2599           
  Branches       658       658           
=========================================
  Hits          2599      2599           

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