[Feature Request] Using VSelect v-slot:selection-display to override the entire selection output

[3dyuval]

Problem to solve

The v-slot:selection slot in VSelect is currently limited to customizing individual selected items. If we need to change how to display selection for the entire state, it's currently possible only using a workaround.
For instance: showing a single label when all items are selected. If we were to implement this feature in our current v-select:selection slot, we'd have to resort to:

<script setup>
import { ref, computed } from 'vue';

    const allItems = ['one', 'two'];
    const itemsSelected = ref(['one', 'two']);
    const allItemsSelected = computed(() => itemsSelected.value.length === allItems.length);

</script>
<template>
  <VSelect v-model="itemsSelected">
    <template #selection="{ item, index }">
      {{ 
        index > 1 && allItemsSelected 
          ? ''
          : index === 0 
          ? allItemsSelected 
            ? 'All items selected' 
            : item 
          : '' 
      }}
    </template>
  </VSelect>
</template>

Proposed solution

Enhance VSelect with a slot or prop for overriding the entire selection template. This would allow to easily define a custom template using v-slot:selection-display and use this to override the entire selection display, providing access to the full itemsSelected array.
Example:

<VSelect v-model="itemsSelected">
  <template #selection-display="{ itemsSelected }">
    {{ itemsSelected.length === allItems.length ? 'All items selected' : itemsSelected.join(', ') }}
  </template>
</VSelect>

[KaelWD]

Similar to #15014