refactor(yaml): inline `testDocumentSeparator()`

[timreichen]

Ref: https://github.com/denoland/std/pull/5749

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.23%. Comparing base (c8fe634) to head (d91160d). Report is 7 commits behind head on main.

Files	Patch %	Lines
yaml/_loader_state.ts	95.00%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5793   +/-   ##
=======================================
  Coverage   96.23%   96.23%           
=======================================
  Files         480      480           
  Lines       38743    38743           
  Branches     5616     5616           
=======================================
  Hits        37286    37286           
  Misses       1416     1416           
  Partials       41       41           

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