fix(VCalendar): Slot day-event not available

[jetze-meilink]

fixes #21341

Markup:

<template>
  <v-app>
    <v-container>
      <VCalendar :events="events">
        <template #day-event>
          <v-chip color="blue">Overidden event slot</v-chip>
        </template>
      </VCalendar>
    </v-container>
  </v-app>
</template>

<script>
  export default {
    name: 'Playground',
    setup () {
      const events = [
        {
          start: new Date(),
          end: new Date(),
          title: 'Test event'
        }
      ]
      return {
        events
      }
    },
  }
</script>

[jetze-meilink]

First time contribution so let me know if there is anything that I have missed.

[jetze-meilink]

The other slots are broken too, it isn't just day-event

You're right. I just fixed those as well.