refactor(yaml): add stringify() method to DumperState

[timreichen]

Changes
This PR moves the stringify() logic inside the DumperState.

Reasoning
This allows the reduction of any types. Other methods can be made private in a future PR.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 96.25%. Comparing base (1bc290b) to head (461efd7).
Report is 2 commits behind head on main.

Files	Patch %	Lines
yaml/_dumper_state.ts	78.57%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5707      +/-   ##
==========================================
- Coverage   96.25%   96.25%   -0.01%     
==========================================
  Files         477      477              
  Lines       38512    38514       +2     
  Branches     5585     5585              
==========================================
+ Hits        37070    37071       +1     
- Misses       1400     1401       +1     
  Partials       42       42              

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[kt3k]

LGTM