fix(VTreeviewItem): add aria-label to toggle button

[docpelux]

Description

Adds an accessible aria-label to the expand/collapse toggle VBtn in VTreeviewItem using the locale composable.
The label toggles between "Expand" and "Collapse" based on the current open state.

Adds corresponding treeview.expand and treeview.collapse locale entries for en and it.

fixes #21585, fixes #22898

Example:

<template>
    <v-treeview
      :items="items"
      open-all
      item-value="id"
    />
</template>
  
<script setup>
  const items = [
    {
      id: 1,
      title: 'Root',
      children: [
        { id: 2, title: 'Child 1' },
        { id: 3, title: 'Child 2', children: [{ id: 4, title: 'Grandchild' }] },
      ],
    },
  ]
</script>

check with Accessibility Insights for Web, should display no issues.

[J-Sek]

closing in favor of #22903