refactor(yaml): add Scanner class

[timreichen]

This PR moves scanner related code from LoaderState into a dedicated Scanner class.

[codecov]

Codecov Report

❌ Patch coverage is 95.06726% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.08%. Comparing base (0aa59e2) to head (8562b37).
⚠️ Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
yaml/_loader_state.ts	95.06%	11 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6958   +/-   ##
=======================================
  Coverage   94.08%   94.08%           
=======================================
  Files         600      600           
  Lines       43540    43591   +51     
  Branches     6991     6993    +2     
=======================================
+ Hits        40965    41014   +49     
- Misses       2520     2522    +2     
  Partials       55       55           

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[kt3k]

LGTM. Thanks!