refactor(yaml): simplify stringifyBlockMapping()

[timreichen]

Changes
This PR simplifies the stringifyBlockMapping() function by using an array and joining it at the end like in stringifyBlockSequence() and moves variable assignments and function calls down so they will not be executed if the loop continues early.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.35%. Comparing base (f86e911) to head (47a5907).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
yaml/_dumper_state.ts	90.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5885   +/-   ##
=======================================
  Coverage   96.34%   96.35%           
=======================================
  Files         479      479           
  Lines       38674    38656   -18     
  Branches     5631     5629    -2     
=======================================
- Hits        37260    37246   -14     
+ Misses       1370     1366    -4     
  Partials       44       44           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[iuioiua]

Thanks