feat(nuxt-img): responsive prop

[pi0]

Support <nuxt-img responsive> to auto generate srcSet sources and sizes based on default values in order to load smaller images in responsive breakpoints. This implementation is based on new $img.getSizes utility.

Alternatives:

• Using <nuxt-picture>
  • It works but has overhead of wrapper div (for placeholder) and
  • Less closer to native img tag for usage
• Using <nuxt-img :srcSet="$img.sizes">
  • Need SSR ctx workaround when `loading=lazy" is enabled for static build
  • More complex code when we need native like img tag and responsive sources
  • More complex usage (= less encouraging to use)
  • Need to also compute sizes for breakpoints

Closes #113