feat: add scoping css prefix option in compiler

[weiliddat]

Add an option in the compiler to change the scoped css class name prefix, partially addresses #570

Super simple, not a callback, just a string. Has a simple test to check if the option works.

Let me know if I should go ahead and extend this if I'm on the right track.

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Tests

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[Conduitry]

Closing in favor of #4377.