refactor(yaml): inline readPlainScalar()

[timreichen]

Ref: #5749

[codecov]

Codecov Report

Attention: Patch coverage is 79.56989% with 19 lines in your changes missing coverage. Please review.

Project coverage is 96.23%. Comparing base (b91817f) to head (12aaece).
Report is 2 commits behind head on main.

Files	Patch %	Lines
yaml/_loader_state.ts	79.56%	18 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5813      +/-   ##
==========================================
- Coverage   96.23%   96.23%   -0.01%     
==========================================
  Files         479      479              
  Lines       38719    38716       -3     
  Branches     5614     5615       +1     
==========================================
- Hits        37262    37259       -3     
  Misses       1416     1416              
  Partials       41       41              

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[iuioiua]

Thank you.