feat(VSlideGroup): add content class to VSlideGroup props

[jcjp]

Description

• Add new props contentClass to VSlideGroup
• Resolves [Feature Request] Styling Limitations Caused by Abstracted Wrappers | Styling #20853

Markup:

<template>
  <v-sheet class="mx-auto" max-width="600">
    <v-slide-group show-arrows content-class="bg-red">
      <v-slide-group-item
        v-for="n in 25"
        :key="n"
        v-slot="{ isSelected, toggle }"
      >
        <v-btn
          :color="isSelected ? 'primary' : undefined"
          class="ma-2"
          rounded
          @click="toggle"
        >
          Options {{ n }}
        </v-btn>
      </v-slide-group-item>
    </v-slide-group>
  </v-sheet>
</template>

<style>
  .bg-red {
    background-color: red;
  }
</style>