feat(isMobilePhone): add en-SZ support

[Musaamanipeace]

Closes #1761

Adds mobile phone validation support for Eswatini (en-SZ).

Added regular expression pattern for Eswatini to src/lib/isMobilePhone.js

Added valid and invalid unit test cases to test/validators/isMobilePhone.js

Updated isMobilePhone supported locales list in README.md

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a79ff98) to head (966f1ee).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2871   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2599      2599           
  Branches       658       658           
=========================================
  Hits          2599      2599           

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