regex added for Senegal sn-SN

[Akilan19]

Validations added for Senegal - Issue https://github.com/validatorjs/validator.js/issues/1761

• isMobilePhone regex added for 'sn-SN'. Reference -Telephone_numbers_in_Senegal
• isPostalCode regex added for 'SN'. Reference
• Relevant unit test added & Readme updated.

CHECKLIST

• [x] PR contains only changes related; no stray files, etc.
• [x] README updated (where applicable)
• [x] Tests written (where applicable)

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (b958bd7) 99.95% compared to head (011da84) 99.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2304      +/-   ##
==========================================
- Coverage   99.95%   99.95%   -0.01%     
==========================================
  Files         107      107              
  Lines        2454     2449       -5     
  Branches      619      619              
==========================================
- Hits         2453     2448       -5     
  Partials        1        1              

Files	Coverage Δ
src/lib/isMobilePhone.js	100.00% <ø> (ø)
src/lib/isPostalCode.js	100.00% <ø> (ø)

... and 3 files with indirect coverage changes

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