feat(VSkeletonLoader): add types prop & reintroduce table-cell

[J-Sek]

• restore missing table-cell type
• tweak table-row styles for consistency
• adapted "table" closer to v2 version
• cleanup unused styles & variables
• add types prop to extend built-ins

resolves #20942
resolves #17506

<template>
  <v-app theme="dark">
    <v-container fluid>
      <v-skeleton-loader type="table" />

      <div class="text-caption">3 columns, 4 rows, via types merged under the defaults</div>
      <v-skeleton-loader
        :types="{
          'table-thead': 'heading@3',
          'table-row': 'table-cell@3',
          'table-tbody': 'table-row-divider@4',
        }"
        type="table"
      />

      <v-data-table
        :headers="headers"
        :items="items"
        :items-per-page="6"
        density="comfortable"
      >
        <template #top>
          <div class="d-flex align-center justify-space-between pa-4">
            <div class="text-headline-small">Table heading</div>
            <div>Some description</div>
          </div>
        </template>
      </v-data-table>
    </v-container>
  </v-app>
</template>

<script setup>
  const headers = Array.from({ length: 6 }, (_, c) => ({ title: `Column ${c + 1}`, key: `c${c}` }))
  const items = Array.from({ length: 12 }, (_, r) => Object.fromEntries(
    headers.map((h, c) => [h.key, `Cell ${r + 1}-${c + 1}`])
  ))
</script>