Format modifier cannot be unset

[Tofandel]

IPX does not have the svg format modifier, this means that if you add in your nuxt config

image: {
  ipx: {
    modifiers: {
      format: 'webp',
    }
  }
}

And then for some svg images you want to override the format to avoid it being converted into an inferior format, none of those solutions will work

<NuxtImage src="my.svg" format="svg">
<!-- Results in ipx f_svg, but ipx doesn't support svg output (even though the original is already svg) -->

<NuxtImage src="my.svg" :format="null">
<!-- Results in ipx f_webp -->

<NuxtImage src="my.svg" :modifiers="{format: null}">
<!-- Results in ipx f_webp -->

Using format auto will also not result in a svg output

So we need a way to either discard the default modifiers or a format "original" in ipx

Added a fix for the first case in unjs/ipx#281