chore: upgrade components to composition API

[Diizzayy]

This PR upgrades the NuxtImg and NuxtPicture components to use the Composition API, and exposes a new useImage composable with the same behavior of the $img utility.

Other changes:

• the nuxt-img component now supports preload

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[AndersNielsen85]

Would be nice adding the example directly to docs. Don't know if that should go to a separate PR? :)

<script lang="ts" setup>
const { generate } = useImage()
const bg = computed(() => {
  const imgUrl = generate('/images/colors.jpg', {
    width: 300,
    height: 300,
    quality: 80
  })
  return `url('${imgUrl}')`
})
</script>```

[pi0]

@AndersNielsen85 I changed back usage to $img = useImage() ($img is a function with attached props)

[Diizzayy]

Thanks for helping on this. Nice changes ❤️

@pi0 no problem, happy to help.