refactor(csv): cleanup use of @std/assert

[iuioiua]

Towards #4865

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

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❗ Current head 053925f differs from pull request most recent head 8c27a68

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Files	Patch %	Lines
csv/_io.ts	0.00%	0 Missing and 1 partial ⚠️
csv/parse.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4976   +/-   ##
=======================================
  Coverage        ?   92.15%           
=======================================
  Files           ?      487           
  Lines           ?    38739           
  Branches        ?     5395           
=======================================
  Hits            ?    35700           
  Misses          ?     2982           
  Partials        ?       57           

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[kt3k]

LGTM