Use classes for style encapsulation, rather than attributes

[Rich-Harris]

Fixes #1118

[codecov-io]

Codecov Report

Merging #1190 into master will decrease coverage by 0.02%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master    #1190      +/-   ##
==========================================
- Coverage   91.69%   91.67%   -0.03%     
==========================================
  Files         126      126              
  Lines        4566     4575       +9     
  Branches     1495     1501       +6     
==========================================
+ Hits         4187     4194       +7     
  Misses        158      158              
- Partials      221      223       +2

Impacted Files	Coverage Δ
src/generators/dom/index.ts	96.17% <ø> (-0.05%)	⬇️
src/css/Stylesheet.ts	91.2% <100%> (ø)	⬆️
...nerators/server-side-rendering/visitors/Element.ts	92.1% <100%> (+0.67%)	⬆️
src/utils/hash.ts	100% <100%> (ø)	⬆️
src/generators/nodes/Element.ts	94.6% <85.71%> (-0.29%)	⬇️
src/generators/nodes/Attribute.ts	85.64% <86.66%> (-0.08%)	⬇️

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