[POC] Refactor PickersDay and DateRangePickerDay components

[noraleonte]

• I have followed (at least) the PR section of the contributing guide.

closes #14753

This is an exploration

I'm exploring the option of creating only one day component used by both simple and range pickers & writing the styles in a way that they are truly separated from each other. i.e. The selected state should not care whether isStartOfPreviewing is true or not. It just applies the styles related to selection.
I reduced the DOM structure to a single element (ButtonBase) and the previewing and highlighting (now renamed to selectedRange) is handled with pseudo elements.

The advantage of this is that we reduce the complexity of customizing this component a lot and we can avoid specificity issues. See new demo vs custom theme from the overview page

A downside is that the width and positioning of the pseudo elements is calculated based on the width and margins of the root => so any customizations of the dimensions or spacing of the root element need to be reflected in the calculation of the dimensions and positionings of the pseudo elements. We can try abstracting some of that and introducing css variables, but I think documenting this should be enough. Plus, it might even make it easier to achieve custom designs where the highlighting and previewing don't connect. (Like the picture)

[mui-bot]

Deploy preview: https://deploy-preview-15921--material-ui-x.netlify.app/

Updated pages:

• docs/data/date-pickers/playground/playground.md

Generated by 🚫 dangerJS against 5ce0b25

[flaviendelangle]

I'll go more in depth on the code changes next year.
But concerning this interface we need to make sure it's properly shaped:

• Should be named PickerDayOwnerState

• Should not extend EnhancedPickersDayProps but only have the properties it actually needs (either to generate the classes or for the variants) and extend PickerOwnerState.

• Every property should be named with the same approach (isLastDayOfTheWeek instead of lastDayOfWeek for example), including the one that come from the props.

• Each property should have some JSDoc explaining the value provided.

Once we have this object clean, it will be easier to see if some properties are not that needed 👌