feat(VToolbar): allow for explicit false for extension

[johnleider]

Motivation and Context

fixes #7317

Markup:

Details

<template>
  <v-container class="pa-md-12">
    <v-toolbar>
      <template #extension>
        Fizzbuzz
      </template>
    </v-toolbar>
    <v-toolbar extended></v-toolbar>
    <v-toolbar :extended="false">
      <template #extension>
        Fizzbuzz
      </template>
    </v-toolbar>
  </v-container>
</template>

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