fix(yaml): handle String instances

[timreichen]

Ref: #5857

Note
It seems weird to handle special treat strings in stringifyNode(). Maybe there is a way to move the evaluation inside Type.predicate like other types in a future PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.32%. Comparing base (890958c) to head (4a860d6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5897   +/-   ##
=======================================
  Coverage   96.32%   96.32%           
=======================================
  Files         483      483           
  Lines       39063    39064    +1     
  Branches     5730     5731    +1     
=======================================
+ Hits        37629    37630    +1     
  Misses       1390     1390           
  Partials       44       44           

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[iuioiua]

Thank you