feat: export overlay function (#1411)

Author: mars-lan

src/index.tsx

@@ -11,6 +11,7 @@ export { default as Provider } from './core/Provider';
 export { default as DefaultTheme } from './styles/DefaultTheme';
 export { default as DarkTheme } from './styles/DarkTheme';
 export { default as shadow } from './styles/shadow';
+export { default as overlay } from './styles/overlay';
 export { default as configureFonts } from './styles/fonts';
 
 import * as Avatar from './components/Avatar/Avatar';