use new css api when available

[nightwing]

using adoptedStylesheets allows to save memory when using ace in shadow dom, and also removes the need for using separate stylesheets in cases when unsafe-inline for css is disabled but data for images is allowed.

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 86.96%. Comparing base (560f5d3) to head (3e8792e).
Report is 112 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib/dom.js	60.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5646      +/-   ##
==========================================
- Coverage   86.97%   86.96%   -0.01%     
==========================================
  Files         594      594              
  Lines       43395    43410      +15     
  Branches     7170     7173       +3     
==========================================
+ Hits        37742    37751       +9     
- Misses       5653     5659       +6     

Flag	Coverage Δ
unittests	86.96% <60.00%> (-0.01%)	⬇️

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