[system] Improve the sx prop IntelliSense

[mnajdova]

This PR improves the typings on the sx prop.

Closes #23219
Closes #15451

It is mostly using the types from https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/styled-system__css/index.d.ts with slight adjustments to our needs

Overriden css prop

Custom css prop

Css prop as function

[mui-pr-bot]

No bundle size changes

Generated by 🚫 dangerJS against 4f2e57b

[eps1lon]

Question: Where is the sx prop actually implemented? It doesn't look like /core/Box is responsible for that.

[mnajdova]

Question: Where is the sx prop actually implemented? It doesn't look like /core/Box is responsible for that.

The unstable_styleFunctionSx which is part of the system is implementing it. This utility is used in the experimentalStyled() - our custom styled() utility and is automatically added on all components created with it - hence the Box and the styled Slider have it.

[eps1lon]

Is there anything more you want to add or is this PR actual ready for review? My question made sense before you moved it form Box to system.

[mnajdova]

Is there anything more you want to add or is this PR actual ready for review? My question made sense before you moved it from Box to system.

It is actual ready for review, was addressing the moving as part of #23599 (comment) didn't seem necessary to create new PR for just that. Sorry for the confusion.