[Box] Add back system props

[mnajdova]

This PR adds back the system props in the Box component. However ,the implementation is a bit different than the previous one. For it to work, I am just extending the value of the sx prop, by adding all system props at the start of the sx object. This means that the value of the sx prop will override these props consistently, which is what we want I believe.

TODO:

• Update perf benchmarks
• Add tests

[mui-pr-bot]

@material-ui/core: parsed: +0.08% , gzip: +0.10%
@material-ui/system: parsed: +3.26% , gzip: +3.47%

Details of bundle changes

Generated by 🚫 dangerJS against 5f9a509

[mnajdova]

Edited: Benchmarks result before merge - https://dev.azure.com/mui-org/Material-UI/_build/results?buildId=22509&view=logs&j=9ef21fd1-5d60-5fa4-f8b2-6dc79e173863&t=516d74c5-15a9-50a0-24d9-dba41a487a2d

[mnajdova]

After the PR is merged, I'll add more benchmarks for the system props too.

[oliviertassinari]

Will we use the same approach to implement the props on the Typography or Grid component?

[mnajdova]

Will we use the same approach to implement the props on the Typography or Grid component?

That's the plan, that's why I've added the extendSxProp utility, it should be pretty much straight forward to add it on any component we may need in the future.

[oliviertassinari]

I assume we will be able to remove this inner component later down the road.

[mnajdova]

Hopefully yes. I had it inlined first, but I needed to add propTypes on it, that’s why I’ve extracted it..