feat(madagascar): phoneNumber & postalCode validations added for Madagascar

[ShivangiRai1310]

Validations added for Madagascar - Issue #1761

• isMobilePhone regex added for 'mg-MG'. Reference -Telephone_numbers_in_Madagascar
• isPostalCode regex added for 'MG'. Reference - List_of_postal_codes
• Relevant unit test added & Readme updated.

Checklist

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

[codecov]

Codecov Report

Merging #2001 (25c1504) into master (6fb8bb5) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #2001   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          103       103           
  Lines         2099      2099           
  Branches       474       474           
=========================================
  Hits          2099      2099           

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

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[rubiin]

Thank you for the contribution