Keep track of amount of duplicate notifications about vue3-notification

[kyvg]

Hi! I added duplicates count to notification item, you can use it like that

<notification>
  <template #body={ item }>
    {{ item.duplicates }}
  </template>
</notification>

Unfortunately, it's not possible to add it to the default body without causing breaking changes, so I decided to just provide that field for custom body for now.

[matsn0w]

Awesome, thanks!