fix: Replace iconv hard dependency with polyfill

[danog]

Technically this could be done by users, but some may not know about the existence of the polyfill, this PR always requires it to avoid this pitfall.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.72%. Comparing base (c0c9671) to head (7b14215).

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #245   +/-   ##
=========================================
  Coverage     71.72%   71.72%           
  Complexity      999      999           
=========================================
  Files            49       49           
  Lines          3158     3158           
=========================================
  Hits           2265     2265           
  Misses          893      893           

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