feat: add useIntersectionObserver

[pikax]

<template> <div> {{ isIntersecting }} <div ref="el"> Element </div> </div> </template> <script> import {ref, watch, reactive} from '@vue/composition-api'; import {useIntersectionObserver} from '../../../';  export default {  name: "intersection-observer-example",  setup(){  const el = ref(null);  const o = useIntersectionObserver(el);  return {  ...o,  el  }  } } </script>