add overflow auto when using anchor

We want to constrain the anchored element within the viewport (minus the
padding if there is some). But without an `overflow: auto`, it would
overflow and the `maxWidth` / `maxHeight` are a bit pointless.

Once you opt-in to `anchor`, then we have to provide styles anywhere.
This improves the UX.

packages/@headlessui-react/src/internal/floating.tsx

@@ -314,6 +314,7 @@ export function FloatingProvider({
       sizeMiddleware({
         apply({ availableWidth, availableHeight, elements }) {
           Object.assign(elements.floating.style, {
+            overflow: 'auto',
             maxWidth: `${availableWidth - padding}px`,
             maxHeight: `${availableHeight - padding}px`,
           })