refactor(yaml): declare-on-use variables

[timreichen]

It seems this doesn't really affect performance but is more readable.

[codecov]

Codecov Report

Attention: Patch coverage is 91.42857% with 3 lines in your changes missing coverage. Please review.

Project coverage is 94.54%. Comparing base (d9a61f8) to head (ff8da5d).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
yaml/_loader_state.ts	91.42%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6594      +/-   ##
==========================================
- Coverage   94.56%   94.54%   -0.02%     
==========================================
  Files         580      580              
  Lines       43778    43771       -7     
  Branches     6489     6492       +3     
==========================================
- Hits        41397    41384      -13     
- Misses       2341     2346       +5     
- Partials       40       41       +1     

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[kt3k]

LGTM. Thanks!