feat(VAppBar): add support for fully-hide scroll-behavior

[johnleider]

resolves #18020

<template>
  <v-app>
    <v-app-bar scroll-behavior="fully-hide" extended />

    <v-main style="height: 500vh;" />
  </v-app>
</template>

<script setup>
  import { ref } from 'vue'
</script>