Add *.css files to the list of sideEffect(ful) files

[Andarist]

This should resolve #761 (comment)

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[chaance]

I'm sure this is fine but I need to research a bit more for my own sake. I'll take care of this ASAP.

[Andarist]

sideEffects flag is actually an allowlist where if you use false as the value you are saying that no files have side-effects. You can use patterns to include files though and this pattern can be seen in the official webpack docs: https://webpack.js.org/guides/tree-shaking/#mark-the-file-as-side-effect-free