fix(VTreeview): make sure toRaw is used on children

[Lisiadito]

Description

resolves #22533
This change makes sure that reactive children can be found in the Map. Before this only worked with non reactive children.

Markup:

<template>
  <div>
    <v-text-field v-model="search" label="Search" />
    <v-treeview
      :items="servers"
      :load-children="loadChildren"
      :search="search"
      density="compact"
      item-title="name"
      item-value="path"
      open-on-click
      return-object
    />
  </div>
</template>

<script setup>
  import { ref, shallowRef } from 'vue'

  const search = shallowRef(null)
  const servers = ref([
    {
      type: 'Server',
      name: 'Server1',
      path: 'server1',
      children: [
        {
          type: 'Directory',
          name: 'Dir1',
          path: 'server1/dir1',
          children: [],
        },
        {
          type: 'Directory',
          name: 'Dir2',
          path: 'server1/dir2',
          children: [],
        },
      ],
    },
  ])

  const loadChildren = item =>
    new Promise(resolve => {
      setTimeout(() => {
        resolve([
          {
            type: 'File',
            name: 'file1.txt',
            size: 0,
            path: item.path + '/file1.txt',
          },
          {
            type: 'File',
            name: 'file2.txt',
            size: 0,
            path: item.path + '/file2.txt',
          }])
      }, 1000)
    })
      .then(data => {
        item.children = data
      })
</script>