feat(VTreeview): Add subtitle & divider slots support

[yuwu9145]

fixes #20781

Description

Markup:

<template>
  <v-treeview :items="items" item-value="id" activatable>
    <template v-slot:title="{ item }"> <b>title - {{ item.title }}</b>  </template>
    <template v-slot:subtitle> subtitle </template>
    <template v-slot:append> append </template>
    <template v-slot:prepend> prepend </template>
    <template v-slot:divider> <v-divider /> </template>
  </v-treeview>
</template>

<script>
  export default {
    data: () => ({
      items: [
        {
          id: 1,
          title: 'Applications :',
          children: [
            { id: 2, title: 'Calendar : app', subtitle: 'CSub' },
            { type: 'divider' },
            { id: 3, title: 'Chrome : app' },
            { type: 'divider' },
            { id: 4, title: 'Webstorm : app' },
          ],
        },
        { type: 'divider' },
        {
          id: 5,
          title: 'Documents :',
          children: [
            {
              id: 6,
              title: 'vuetify :',
              children: [
                {
                  id: 7,
                  title: 'src :',
                  children: [
                    { id: 8, title: 'index : ts' },
                    { type: 'divider' },
                    { id: 9, title: 'bootstrap : ts' },
                  ],
                },
              ],
            },
            {
              id: 10,
              title: 'material2 :',
              children: [
                {
                  id: 11,
                  title: 'src :',
                  children: [
                    { id: 12, title: 'v-btn : ts' },
                    { id: 13, title: 'v-card : ts' },
                    { id: 14, title: 'v-window : ts' },
                  ],
                },
              ],
            },
          ],
        },
        { type: 'divider' },
        {
          id: 15,
          title: 'Downloads :',
          children: [
            { id: 16, title: 'October : pdf' },
            { id: 17, title: 'November : pdf' },
            { id: 18, title: 'Tutorial : html' },
          ],
        },
        { type: 'divider' },
        {
          id: 19,
          title: 'Videos :',
          children: [
            {
              id: 20,
              title: 'Tutorials :',
              children: [
                { id: 21, title: 'Basic layouts : mp4' },
                { id: 22, title: 'Advanced techniques : mp4' },
                { id: 23, title: 'All about app : dir' },
              ],
            },
            { id: 24, title: 'Intro : mov' },
            { id: 25, title: 'Conference introduction : avi' },
          ],
        },
      ],
    }),
  }
</script>

[yuwu9145]

title, subtitle, append, prepend & divider slots are supported now.

As for "header" & "subheader", I'm happy to implement them when feature requests come. At this stage and in my opinion, they are less likely to be used in treeview context.