[Flare] Add FocusWithin responder (#16152)

FocusWithin is implemented as a separate responder, which keeps both focus
responders simple and allows for easier composition of behaviours.

packages/react-events/docs/FocusWithin.md

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+# FocusWithin
+
+The `FocusWithin` module responds to focus and blur events on its child. Focus events
+are dispatched for all input types, with the exception of `onFocusVisibleChange`
+which is only dispatched when focusing with a keyboard.
+
+Focus events do not propagate between `FocusWithin` event responders.
+
+```js
+// Example
+const Button = (props) => {
+  const [ focusWithin, updateFocusWithin ] = useState(false);
+  const [ focusWithinVisible, updateFocusWithinVisible ] = useState(false);
+
+  return (
+    <FocusWithin
+      onFocusWithinChange={updateFocusWithin}
+      onFocusWithinVisibleChange={updateFocusWithinVisible}
+    >
+      <button
+        children={props.children}
+        style={{
+          ...(focusWithin && focusWithinStyles),
+          ...(focusWithinVisible && focusWithinVisibleStyles)
+        }}
+      >
+    </FocusWithin>
+  );
+};
+```
+
+## Types
+
+```js
+type FocusEvent = {
+  target: Element,
+  type: 'focuswithinchange' | 'focuswithinvisiblechange'
+}
+```
+
+## Props
+
+### disabled: boolean = false
+
+Disables all `FocusWithin` events.
+
+### onFocusWithinChange: boolean => void
+
+Called once the element or a descendant receives focus, and once focus moves
+outside of the element.
+
+### onFocusWithinVisibleChange: boolean => void
+
+Called once the element or a descendant is focused following keyboard
+navigation, and once focus moves outside of the element. This can be used to
+display focus styles only when the keyboard is being used to focus within the
+element's subtree.