feat(VTreeview): add slot to replace expand toggle btn

[johnleider]

Description

Markup:

<template>
  <v-treeview
    v-model="model"
    :items="items"
    collapse-icon="mdi-chevron-down"
    expand-icon="mdi-chevron-right"
    item-value="id"
    select-strategy="classic"
    selectable
  >
    <template #toggle="{ props: toggleProps }">
      <v-btn v-bind="toggleProps">
        <v-icon size="16" />
      </v-btn>
    </template>

    <template #title="{ item }">
      <span class="text-caption">{{ item.title }}</span>
    </template>
  </v-treeview>
</template>
<script setup>
  import { computed, ref } from 'vue'

  const model = ref()

  const GetChildren = id => {
    const nodes = []
    for (let i = 0; i < 5; i++) {
      nodes.push({
        id: `${id}-${i}`,
        title: `Example child item ${i}`,
      })
    }
    return nodes
  }

  const items = computed(() => {
    const nodes = []
    for (let i = 0; i < 2; i++) {
      nodes.push({
        id: `${i}`,
        title: `Example item ${i}`,
        children: GetChildren(i),
      })
    }
    return nodes
  })
</script>