Placeholder slot for <NuxtImg>

[DamianGlowala]

I've been thinking of how to achieve more sophisticated placeholders using <NuxtImg> than what the placeholder prop allows for. Being able to define the placeholder in a slot would allow for greater flexibility:

<NuxtImg>
  <template #placeholder>
    <SkeletonLoader />
  </template>
</NuxtImg>

If this is a desirable addition, I am willing to implement it.