Rework svelte & clean up rollup integrations

[tivac]

• Svelte no longer has a custom rollup integration, use modular-css-rollup instead
• modular-css-rollup now supports a common option that will handle outputting any CSS that was removed from chunks due to treeshaking.
• modular-css-rollup accepts a new processor option that is expected to be a fully-configured & instantiated instance of Processor from modular-css-core.
• Updated svelte docs

[codecov]

Codecov Report

Merging #430 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #430      +/-   ##
==========================================
- Coverage    99.1%   99.08%   -0.02%     
==========================================
  Files          36       31       -5     
  Lines         779      765      -14     
  Branches      113      114       +1     
==========================================
- Hits          772      758      -14     
  Misses          7        7

Impacted Files	Coverage Δ
packages/svelte/svelte.js	100% <100%> (ø)	⬆️
packages/svelte/src/markup.js	100% <100%> (ø)
packages/rollup/rollup.js	100% <100%> (ø)	⬆️
packages/svelte/src/style.js	100% <100%> (ø)
packages/test-utils/read.js
packages/test-utils/namer.js
packages/test-utils/exists.js
packages/test-utils/prefix.js
packages/test-utils/relative.js
... and 1 more

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