Enable Material 3 on provider_shopper (#1627)

Co-authored-by: Brett Morgan <brettmorgan@google.com>

provider_shopper/lib/common/theme.dart

@@ -5,7 +5,8 @@
 import 'package:flutter/material.dart';
 
 final appTheme = ThemeData(
-  primarySwatch: Colors.yellow,
+  colorSchemeSeed: Colors.yellow,
+  useMaterial3: true,
   textTheme: const TextTheme(
     displayLarge: TextStyle(
       fontFamily: 'Corben',

provider_shopper/lib/screens/cart.dart

@@ -85,7 +85,7 @@ class _CartTotal extends StatelessWidget {
                 builder: (context, cart, child) =>
                     Text('\$${cart.totalPrice}', style: hugeStyle)),
             const SizedBox(width: 24),
-            TextButton(
+            FilledButton(
               onPressed: () {
                 ScaffoldMessenger.of(context).showSnackBar(
                     const SnackBar(content: Text('Buying not supported yet.')));