fix: Represent string literal types as unions of a single element

[pan-kot]

Required for cloudscape-design/components#2469 where we introduce a new union type with a single element for now.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.35%. Comparing base (03f2422) to head (69d2388).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
+ Coverage   95.21%   95.35%   +0.14%     
==========================================
  Files          15       15              
  Lines         418      431      +13     
  Branches      144      147       +3     
==========================================
+ Hits          398      411      +13     
  Misses         20       20              

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