feat(transitions): add root variable

[johnleider]

Motivation and Context

closes #6470

Markup:

<template>
  <v-app>
    <div class="ma-4 pa-4">
      <v-list :items="items" />
    </div>
  </v-app>
</template>

<script setup>
  const items = [
    { title: 'Item 1', children: [{ title: 'Item 1.1' }] },
    { title: 'Item 2' },
    { title: 'Item 3' },
  ]
</script>