[system] Reexport Pigment CSS from index file

[siriwatknp]

Issue

The changes fixe theme augmentation issue with the latest beta:

Change

Remove the @mui/material-pigment-css/theme and reexport from the index directly. Using theme subfolder adds unnecessary complexity and require user to import type {} from @mui/material-pigment-css/theme first.

The change require users to set up TypeScript as below (documented in the migration guide):

declare module "@mui/material-pigment-css" {
  interface ThemeArgs {
    theme: Theme
  }
}

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

[mui-bot]

Netlify deploy preview

• docs/data/material/migration/migrating-to-v6/migrating-to-pigment-css.md

Bundle size report

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[siriwatknp]

This requires import {} from '@mui/material-pigment-css/theme' at least once to be able to augment correctly. For example,

import {} from '@mui/material-pigment-css/theme';
import { Theme } from '@mui/material/styles';

declare module "@mui/material-pigment-css/theme" {
  interface ThemeArgs {
    theme: Theme
  }
}

I wonder if it makes more sense to augment the theme from @mui/material-pigment-css instead of theme subpath. cc @brijeshb42

[siriwatknp]

@brijeshb42 Verified that the change is working as expected.

[DiegoAndai]

Some of these seem to have leaked from #43217?

[mnajdova]

I reverted the changes on the migration-to-v6 which are not related to this PR. It looks good to me

[siriwatknp]

I reverted the changes on the migration-to-v6 which are not related to this PR. It looks good to me

Thanks @mnajdova

[oliviertassinari]

Suggested change

	### Typescript
	### TypeScript

Fixing this in #43751