TextInput: Switch to `useMergeRefs`
TextInput: Switch to `useMergeRefs`

Summary:
Switches `TextInput` from `setAndForwardRefs` to `useMergeRefs` (and `useCallback`).

Instead of creating a new `_setNativeRef` function on every render, this will now only create a new ref function when either `mostRecentEventCount` changes, `viewCommands` (i.e. `props.multiline` on iOS) changes, or when `props.forwardedRef` changes.

When a text input is being edited, `mostRecentEventCount` will probably change. But when an unfocused `TextInput` is being updated because a parent is being updated, we will now no longer unnecessarily create a new `ref`. The observable behavior of this is that any `ref` supplied onto `TextInput` will now be invoked less frequently.

Changelog:
[General][Changed] Any `ref` set on `TextInput` will now be updated less frequently (when the underlying `ref` has not changed).

Reviewed By: sammy-SC

Differential Revision: D41191439

fbshipit-source-id: c69a061317c51ad6c6ca8acd116539e0f24a1d08

facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native