feat: add postal code validation for TR, PH, MA, and VN

[realkeshav08]

Description

This PR adds support for validating postal codes from four additional countries inside the isPostalCode validator:

1. Turkey (TR): 5 digits (e.g. 34000)
2. Philippines (PH): 4 digits (e.g. 1000)
3. Morocco (MA): 5 digits (e.g. 20000)
4. Vietnam (VN): 5 digits (e.g. 10000)

Changes Made

• Added regex patterns for TR, PH, MA, and VN to the patterns list in src/lib/isPostalCode.js.
• Added corresponding unit test fixtures (both valid and invalid test cases) in test/validators.test.js.

Verification

Ran tests locally using npm test and verified that:

• All 318 test suites pass successfully.
• 100% statement and line coverage is maintained across all modified sections.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fdc788) to head (05a3c18).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2740   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2587      2587           
  Branches       656       656           
=========================================
  Hits          2587      2587           

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