refactor(yaml): inline mergeMappings()

[timreichen]

Ref: #5749

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.23%. Comparing base (74a92c4) to head (fafd529).
Report is 3 commits behind head on main.

Files	Patch %	Lines
yaml/_loader_state.ts	95.45%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5777   +/-   ##
=======================================
  Coverage   96.23%   96.23%           
=======================================
  Files         481      481           
  Lines       38745    38744    -1     
  Branches     5616     5617    +1     
=======================================
+ Hits        37285    37287    +2     
+ Misses       1419     1416    -3     
  Partials       41       41           

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[iuioiua]

Thank you