fix: use self instead of window or document for worker context

[krissvaa]

Adjust the library to be used in the Service Worker context without DOM global variables

Fixes vaadin/hilla#2867

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.31%. Comparing base (c67a967) to head (aaed4b3).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #16   +/-   ##
=======================================
  Coverage   98.30%   98.31%           
=======================================
  Files           3        3           
  Lines         650      651    +1     
  Branches       37       37           
=======================================
+ Hits          639      640    +1     
  Misses         11       11           

Flag	Coverage Δ
unittests	98.31% <ø> (+<0.01%)	⬆️

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