feat: Provide height with sizes attribute

[wokalek]

Described a feature with use case in this issue #900

Now you can provide height with sizes attribute like this:

md:114px_114px lg:138px_138px

or this, short version:

md:114_114 lg:138_138

Examples:

Old output:
sizes="md:114px lg:138px" with fit="cover" (does nothing, because it's generate w_{width} modifier)

<img 
  src="/_ipx/w_276/images/colors.jpg"
  sizes="(max-width: 768px) 114px, 138px"
  srcset="/_ipx/w_114/images/colors.jpg 114w, /_ipx/w_138/images/colors.jpg 138w, /_ipx/w_228/images/colors.jpg 228w, /_ipx/w_276/images/colors.jpg 276w"
>

New output:
sizes="md:114_114 lg:138_138" with fit="cover"

<img 
  src="/_ipx/fit_cover&amp;s_276x276/images/colors.jpg"
  sizes="(max-width: 768px) 114px, 138px"
  srcset="/_ipx/fit_cover&amp;s_114x114/images/colors.jpg 114w, /_ipx/fit_cover&amp;s_138x138/images/colors.jpg 138w, /_ipx/fit_cover&amp;s_228x228/images/colors.jpg 228w, /_ipx/fit_cover&amp;s_276x276/images/colors.jpg 276w"
>

And now modifiers working well!

fit="outside" too! Because this is resize operation now, not only width limit.

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