convertSize to webp as an option

[zehawki]

Is your feature request related to a problem? Please describe.
Current behavior - PNG files over convertSize value will be converted to JPEGs (https://github.com/fengyuanchen/compressorjs#convertsize)

Describe the solution you'd like
Suggested enhancement - Add a feature (and flag) which will convert PNGs to WEBP instead of JPEG.

Additional context
This is to retain PNG transparency while reducing the image size

[fengyuanchen]

Thanks for your suggestion, I will think about it.