refactor(yaml): simplify and rename dropEndingNewline()

[timreichen]

• renames dropEndingNewline() to trimTrailingNewline()
• uses .at(-1)
• adds tests

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.81%. Comparing base (5cccc4b) to head (85fbe41).

Files	Patch %	Lines
yaml/_dumper.ts	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5336      +/-   ##
==========================================
+ Coverage   95.80%   95.81%   +0.01%     
==========================================
  Files         458      458              
  Lines       37871    37869       -2     
  Branches     5562     5562              
==========================================
+ Hits        36283    36285       +2     
+ Misses       1548     1544       -4     
  Partials       40       40              

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