feat(isLicensePlate): added License Plate Regex for en-SG

[Sabarinathan07]

Added validation for Singapore license plates.

The below are some valid Singapore license plates. Singapore license plates follow a format of three letters, four numbers, and a letter.
SGX 1234 A
SMD1234Z

Refrences:
Wikipedia/Vehicle_registration_plates_of_Singapore

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 99.95%. Comparing base (83d6ffd) to head (b0e8113).
Report is 2 commits behind head on master.

❗ Current head b0e8113 differs from pull request most recent head 2cfa3c3. Consider uploading reports for the commit 2cfa3c3 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##            master    #2333      +/-   ##
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- Coverage   100.00%   99.95%   -0.05%     
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  Files          108      107       -1     
  Lines         2482     2450      -32     
  Branches       627      619       -8     
===========================================
- Hits          2482     2449      -33     
- Partials         0        1       +1     

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