Base stylesheet.id on CSS contents

[Rich-Harris]

Follow-up to #1105 that causes CSS scoping to be based on styles alone, but also preserves the previous ast.hash behaviour. The mysterious test failure is still present.

[codecov-io]

Codecov Report

Merging #1119 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1119   +/-   ##
=======================================
  Coverage   91.47%   91.47%           
=======================================
  Files         125      125           
  Lines        4518     4518           
  Branches     1457     1457           
=======================================
  Hits         4133     4133           
  Misses        163      163           
  Partials      222      222

Impacted Files	Coverage Δ
src/parse/index.ts	88.04% <ø> (ø)	⬆️
src/utils/hash.ts	100% <ø> (ø)
src/css/Stylesheet.ts	89.9% <100%> (ø)	⬆️

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