refactor(yaml): rename DumperState methods

[timreichen]

Changes
This PR renames DumperState methods fro write*() to stringify*()

Reasoning
#5683 removed the dump property. All these functions now turn passed data into a string and return it.

[iuioiua]

Can you please add explicit return types?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.28%. Comparing base (d6fde64) to head (6e7cd31).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5738      +/-   ##
==========================================
- Coverage   96.28%   96.28%   -0.01%     
==========================================
  Files         474      474              
  Lines       38387    38383       -4     
  Branches     5578     5578              
==========================================
- Hits        36962    36957       -5     
- Misses       1383     1384       +1     
  Partials       42       42              

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