refactor(VField): default slot fallback

[johnleider]

A small QOL change for working with VField.

TODO: doc updates

Description

Markup:

<template>
  <v-app>
    <v-container>
      <v-input>
        <v-field
          append-icon="$vuetify"
          label="Foobar"
          prepend-icon="$vuetify"
        />

        <v-menu activator="parent" eager>
          <v-list v-model:selected="selected">
            <v-list-item title="Foobar" value="one" />
            <v-list-item title="Foobar" value="two" />
          </v-list>
        </v-menu>
      </v-input>
    </v-container>
  </v-app>
</template>

<script setup>
  import { ref } from 'vue'

  const file = ref()
  const isFocused = ref(false)

  const selected = ref([])
</script>