enhancement: string as the default export if the preset only defines srcset

[ElMassimo]

Description 📖

Currently the default export for preset imports is ImageSource[]. This allows presets to define additional source tags, with different formats, conditional widths and densities, etc.

For presets that define a single ImageSource that only contains srcsets, it would be convenient for the default export to be a string.

That would make it work out of the box in img and source tags in HTML and Vue SFCs, where src and srcset are already extracted as imports.