fix(next/image): improve warning when `fill` and `sizes="100vw"` (#61949)
Previously, this error was confusing because it made it sound like the
`sizes` prop was missing. This was because the default value of `sizes`
is `100vw` so the previous code couldn't tell the different between
implicit vs explicit `100vw`.
This PR changes the code to read the input value from the `sizes` prop
and prints a better warning.
Fixes NEXT-2441