dillon1000/react

Offscreen add attach (#25603)

`Offscreen.attach` is imperative API to signal to Offscreen that its
updates should be high priority and effects should be mounted. Coupled
with `Offscreen.detach` it gives ability to manually control Offscreen.
Unlike with mode `visible` and `hidden`, it is developers job to make
sure contents of Offscreen are not visible to users.
`Offscreen.attach` only works if mode is `manual`.

Example uses:
```jsx
let offscreenRef = useRef(null);
<Offscreen mode={'manual'} ref={offscreenRef)}>
  <Child />
</Offscreen>

// ------

// Offscreen is attached by default. 
// For example user scrolls away and Offscreen subtree is not visible anymore.
offscreenRef.current.detach();


// User scrolls back and Offscreen subtree is visible again.
offscreenRef.current.attach();
```

Co-authored-by: Andrew Clark <[email protected]>