[material-ui] Fix regression on theme.vars.shape module augmentation

[siriwatknp]

Reported in #47007 (comment)

New doc page: https://deploy-preview-47164--material-ui.netlify.app/material-ui/customization/shape/

Root cause

Before the fix (#47007), the ThemeVars is using Theme['shape'], so the module augmentation on Theme also applied to ThemeVars
However, it's a cyclic inference which probably one of the root cause for TS performance spike.

The fix (#47007) removes the cyclic inference but causes the regression.

Fix

Both Theme and ThemeVars need to rely on a new interface Shape that extends the @mui/system's interface.

User now needs to augment the Shape instead of the Theme.

declare module "@mui/material/styles" {
  interface Shape {
    extraShape: string
  }
}

• Test added

---

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

[mui-bot]

Netlify deploy preview

• docs/data/material/customization/shape/shape.md

Bundle size report

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

Generated by 🚫 dangerJS against 63cf433

[mj12albert]

Looks good ~ tested with the Vite example repo: https://stackblitz.com/edit/github-nztqdw3a?file=src%2FApp.tsx