feat(isLicensePlate): add greek license plate validation

[phxgg]

Add greek license plate validation for the new and legacy plates.

Checklist

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

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 100.00%. Comparing base (6f436be) to head (ec14ae2). :warning: Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2614   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2536      2537    +1     
  Branches       642       642           
=========================================
+ Hits          2536      2537    +1     

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