[core] v6 emotion-less css & styled prototype

[romgrk]

Prototype for emotion-less styling.

DataGrid PR: mui/mui-x#15160

[romgrk]

@siriwatknp Sidenote but this is part of what can be optimized, right now the whole styleOverrides is re-processed each render for each slot, even if overridesResolver only selects a small slice on the styleOverrides. It should be stable as long as theme is stable, so we should be able to pre-serialize & memo this. Same for theme variants.

[mui-bot]

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and 180 more changes

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