feat(isAlpha): added Estonian (et-EE) support to alpha and alphanumeric validation

[kurbar]

Added support for Estonian (et-EE) in isAlpha and isAlphanumeric validation. Estonian required characters added: Š, Ž, Õ, Ä, Ö, Ü.

Checklist

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (3847c6f) to head (f3e8549).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2562   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2535      2535           
  Branches       641       641           
=========================================
  Hits          2535      2535           

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[WikiRik]

Can you add a test for this and provide a resource for why these characters are required?