[POC] Switch between MD2 and MD3

[mnajdova]

This PR experiments how we can add support for MD3 while keeping MD2 alongside. The idea is to provide two themes, and for now keep MD2 by default. In v7 we can switch the default to MD3.

In order to make sure we won't break the previous users' style overrides, we can isolate the styles for MD3 completely by providing different slots components. Each component can use the useIsMD3 hook, and based on it use the correct styled component (see the Button implementation in the PR).

Preview: https://deploy-preview-42904--material-ui.netlify.app/experiments/md3/

[mui-bot]

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https://deploy-preview-42904--material-ui.netlify.app/

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[siriwatknp]

I like the idea of switching the slots. It seems to be the viable method without huge breaking change. However, I don't agree with using useIsMd3 to check inside each component because it will increase the bundle size significantly (non tree-shaking) and I think it's hard to maintain in the future.

Instead I think we have to extract specific MD2 styles out of the component to MD2Theme, meaning the default Button will display as plain button.

import Button from '@mui/material/Button';

<Button>test</Button> // this button will be a plain button without MD2 specific, like Ripple.

Then create a theme and slots for MD2 and MD3:

import { extendMd2Theme, Md2Slots } from '@mui/material/md2';

<CssVarsProvider theme={extendTheme()}>
  <DefaultPropsProvider value={MD2Slots}>
     …app
  </DefaultPropsProvider>
</CssVarsProvider>

The MD2Slots could look like this:

{ MuiButton: { defaultProps: { slots: { ripple: TouchRipple } }} }

Same for MD3:

import { extendMd3Theme, Md3Slots } from '@mui/material/md3';

<CssVarsProvider theme={extendMd3Theme()}>
  <DefaultPropsProvider value={Md3Slots}>
     …app
  </DefaultPropsProvider>
</CssVarsProvider>

{ MuiButton: { defaultProps: { slots: { ripple: null } }} }

It's quite similar to MUI X Data Grid slots override but I need to do a POC first.

[mnajdova]

I would advocate for keeping this internal details, regardless of how we implement it internally, slots, theme etc, the end-user experience should be that they need to change the theme. Let's create a POC, we can hide the implementation details about the default props as part of the theme.

[mnajdova]

I am closing this experiment, we'll have it anyway if wee need to come back at it.