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)