process code in @supports queries with cascade: false

[leereamsnyder]

with the cascade: false option, CSS in @supports blocks was getting removed

<div class='maybe-grid'>something with a nice layout</div>

<style>
  @supports (display: grid) {
    .maybe-grid {
      display: grid;
    }
  }
</style>

You would expect to get the code within the block e.g.

@supports (display: grid) {.maybe-grid[svelte-xyz]{display:grid}}

but you would get

@supports (display: grid) {}

[codecov-io]

Codecov Report

Merging #1200 into master will increase coverage by 0.05%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #1200      +/-   ##
==========================================
+ Coverage   91.63%   91.69%   +0.05%     
==========================================
  Files         127      127              
  Lines        4567     4573       +6     
  Branches     1503     1505       +2     
==========================================
+ Hits         4185     4193       +8     
+ Misses        160      158       -2     
  Partials      222      222

Impacted Files	Coverage Δ
src/css/Stylesheet.ts	91.2% <71.42%> (+1.2%)	⬆️

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[Rich-Harris]

Excellent, thank you!