feat(isPostalCode): add BD locale

[tanvirrb]

This pull request includes updates to the README.md documentation, the isPostalCode validation logic, and the associated tests to add support for Bangladeshi postal codes.

Documentation update:

• README.md: Added 'BD' (Bangladesh) to the list of supported locales for the isPostalCode function.

Validation logic update:

• src/lib/isPostalCode.js: Added a new regular expression pattern for Bangladeshi postal codes to the patterns object.

Test update:

• test/validators.test.js: Added test cases for Bangladeshi postal codes, including both valid and invalid examples.

Reference: Bangladehi postal code list in Wikipedia

Checklist

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (bdb8f9e) to head (50765f8).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2551   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2535      2535           
  Branches       641       641           
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  Hits          2535      2535           

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[WikiRik]

Looks good! Just a small suggestion to simplify the regex