[pigment-css] Make @pigment-css/react as peer dep

This change helps in module resolution across different package
managers, especially pnpm, which expects the imports to be from direct
dependencies.
During sx prop transform, pigment-css plugin adds -

```js
import { sx } from '@pigment-css/react';
```

which would throw an error when using pnpm since @pigment-css/react was
not a direct dependency of the user's application.

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

@@ -31,15 +31,15 @@ First, install the Material UI wrapper package for Pigment CSS:
 <codeblock storageKey="package-manager">
 
 ```bash npm
-npm install @mui/material-pigment-css
+npm install @mui/material-pigment-css @pigment-css/react
 ```
 
 ```bash pnpm
-pnpm add @mui/material-pigment-css
+pnpm add @mui/material-pigment-css @pigment-css/react
 ```
 
 ```bash yarn
-yarn add @mui/material-pigment-css
+yarn add @mui/material-pigment-css @pigment-css/react
 ```
 
 </codeblock>

packages/mui-material-pigment-css/package.json

@@ -40,8 +40,10 @@
   },
   "dependencies": {
     "@babel/runtime": "^7.26.0",
-    "@mui/system": "workspace:*",
-    "@pigment-css/react": "0.0.27"
+    "@mui/system": "workspace:*"
+  },
+  "peerDependencies": {
+    "@pigment-css/react": "^0.0.27"
   },
   "sideEffects": false,
   "publishConfig": {