feat(isLicensePlate): support for Serbian license plates

[theteladras]

Adding support for validation of Serbian vehicles license plates.

Reference can be found here: https://en.wikipedia.org/wiki/Vehicle_registration_plates_of_Serbia

Diplomatic plates are not included as I do not see a use case for it. If you do believe its something to be added, do let me know and will make the required adjustments.

Checklist

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

[codecov[bot]]

Codecov Report

Merging #2038 (a8efc3f) into master (86a07ba) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #2038   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          104       104           
  Lines         2203      2204    +1     
  Branches       477       477           
=========================================
+ Hits          2203      2204    +1     

Impacted Files	Coverage Δ
src/lib/isLicensePlate.js	100.00% <100.00%> (ø)

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[theteladras]

@profnandaa Since no activity on this, I'll be closing this PR next week.