docs(isAlpha,isAlphanumeric): add th-TH to README

[WikiRik]

Closes #1757
th-TH are accepted and tested locales in isAlpha and isAlphanumeric but are not in the README.

Checklist

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

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (984fcdc) compared to base (6dba289).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2160   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          105       105           
  Lines         2334      2334           
  Branches       586       586           
=========================================
  Hits          2334      2334           

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[profnandaa]

Add this in a the upcoming combined fix commit just to simplify things.