fix(VDataTableRow): provide correct defaults for special slots

[johnleider]

When using the special item.data-table-select and item.data-table-expand slots, would always return at:

and never actually get to the code that called slots['item.data-table-expand'] or select.

Since I was fixing that, I brought the slots in line with how we handle defaults in other components, so that you can actually modify these with relative ease now.

Description

Markup:

<template>
  <v-container class="pa-md-12">
    <v-data-table :items="items" show-expand show-select>
      <template #item.data-table-select="{ props: selectProps }">
        <v-checkbox-btn v-bind="selectProps" />
      </template>

      <template #item.data-table-expand="{ props: expandProps }">
        <v-btn v-bind="expandProps" />
      </template>

      <template #expanded-row="{ columns, item }">
        <tr>
          <td :colspan="columns.length">
            I'm expanded for item {{ item.id }}!
          </td>
        </tr>
      </template>
    </v-data-table>
  </v-container>
</template>

<script setup>
  const items = [
    { id: 'John', age: 42 },
    { id: 'Jane', age: 36 },
    { id: 'Joe', age: 30 },
  ]
</script>