Add utility functions: useFocusAtom, useSelectValue

Instead of wrapping focusAtom with useAtom, and selectAtom with useAtomValue, I think the following would be nice:

useFocusAtom

import { useFocusAtom } from "jotai/optics";

function SomeComponent(props) {
  const [z, setZ] = useFocusAtom(x, (y) => y.prop("someKey"))
  return <></>
}

useSelectValue

import { useSelectValue } from "jotai/utils";

function SomeComponent(props) {
  const z = useSelectValue(x, (y) => y.someKey))
  return <></>
}

I guess the implementations would be something like this:

export function useFocusAtom(anAtom, keyFn) {
  return useAtom(focusAtom(anAtom, keyFn))
}

export function useSelectValue(anAtom, keyFn) {
  return useAtomValue(selectValue(anAtom, keyFn))
}

[LuckyPigeon]

@dai-shi I think I can support the documentation, let me do some research first.

[dai-shi]

closed by #923