Properly await async polyfill function to avoid race condition

[audiodude]

Fixes #2054

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 74.88%. Comparing base (372f0c7) to head (dbff5d9).

Files	Patch %	Lines
src/mwoffliner.lib.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2055      +/-   ##
==========================================
+ Coverage   74.38%   74.88%   +0.50%     
==========================================
  Files          41       41              
  Lines        3146     3146              
  Branches      689      689              
==========================================
+ Hits         2340     2356      +16     
+ Misses        686      671      -15     
+ Partials      120      119       -1     

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