dillon1000/react

Define `HostInstance` type for React Native (#31101)

## Summary

Creates a new `HostInstance` type for React Native, to more accurately
capture the intent most developers have when using the `NativeMethods`
type or `React.ElementRef<HostComponent<T>>`.

Since `React.ElementRef<HostComponent<T>>` is typed as
`React.AbstractComponent<T, NativeMethods>`, that means
`React.ElementRef<HostComponent<T>>` is equivalent to `NativeMethods`
which is equivalent to `HostInstance`.


## How did you test this change?

```
$ yarn
$ yarn flow fabric
```