fix(webfonts): support CSSGroupingRule

[egedarici]

This allows @font-face defined in https://github.com/layer rules.

[biiibooo]

👋 @egedarici

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[codecov]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes missing coverage. Please review.

Project coverage is 66.50%. Comparing base (d9b2fcf) to head (e0054c1).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/embed-webfonts.ts	69.23%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #522   +/-   ##
=======================================
  Coverage   66.50%   66.50%           
=======================================
  Files          10       10           
  Lines         612      612           
  Branches      150      150           
=======================================
  Hits          407      407           
  Misses        144      144           
  Partials       61       61           

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