Added exercises

step2-06/exercise/src/components/TodoApp.tsx

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+import React from 'react';
+import { Stack } from 'office-ui-fabric-react';
+import { TodoFooter } from './TodoFooter';
+import { TodoHeader } from './TodoHeader';
+import { TodoList } from './TodoList';
+
+export const TodoApp = () => {
+  return (
+    <Stack horizontalAlign="center">
+      <Stack style={{ width: 400 }} gap={25}>
+        <TodoHeader />
+        <TodoList />
+        <TodoFooter />
+      </Stack>
+    </Stack>
+  );
+};