fix: correctly migrate reactive assignment with member expression

From https://github.com/sveltejs/kit/pull/12519#issuecomment-2258840595

packages/svelte/src/compiler/migrate/index.js

@@ -412,7 +412,11 @@ const instance_script = {
 			const ids = extract_identifiers(node.body.expression.left);
 			const bindings = ids.map((id) => state.scope.get(id.name));
 			const reassigned_bindings = bindings.filter((b) => b?.reassigned);
-			if (reassigned_bindings.length === 0 && !bindings.some((b) => b?.kind === 'store_sub')) {
+			if (
+				reassigned_bindings.length === 0 &&
+				!bindings.some((b) => b?.kind === 'store_sub') &&
+				node.body.expression.left.type !== 'MemberExpression'
+			) {
 				// $derived
 				state.str.update(
 					/** @type {number} */ (node.start),

packages/svelte/tests/migrate/samples/effects/input.svelte

@@ -10,4 +10,5 @@
 		console.log('bar');
 	}
 	$: $count = 1;
+	$: foo.bar = 1;
 </script>

packages/svelte/tests/migrate/samples/effects/output.svelte

@@ -18,4 +18,7 @@
 	run(() => {
 		$count = 1;
 	});
+	run(() => {
+		foo.bar = 1;
+	});
 </script>