[Refactor] Helper for reading yaml header

[MichaelChirico]

This avoids the way close() is done. We could also use local() but this code is already pretty nested, better to off-load to a helper IMO.

A broader question of whether we might just deprecate yaml=TRUE is left unaddressed, for now.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.97%. Comparing base (291a711) to head (811e4ff).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7552   +/-   ##
=======================================
  Coverage   98.97%   98.97%           
=======================================
  Files          87       87           
  Lines       16733    16733           
=======================================
  Hits        16561    16561           
  Misses        172      172           

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[github-actions]

No obvious timing issues in HEAD=yaml-header-helper

Generated via commit 811e4ff

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	3 minutes and 0 seconds
Installing different package versions	22 seconds
Running and plotting the test cases	4 minutes and 6 seconds