[RFC] Add a new syntax for srcset

[lilith]

In srcset attributes, length matters. Short, comma-delimited directives can be valuable There's probably a better key, but we can add that later

?srcset=webp90,2x,sharp15,480w

• [0-9]+w and [0-9]+h will set width and height values.
• crop will set mode=crop, pad will set mode=pad, the default will otherwise be max
• [0-9.]+x will set zoom=[value]
• `jpg[0-9][0-9] will use format=jpeg&jpeg.quality=x
• `webp[0-9][0-9] will use format=webp&webp.quality=x
• png[0-9][0-9]
• png
• webp-lossless,
• gif
• sharp[0-9][0-9] will apply f.sharpen=v