Platform image helper

[alexgibson]

Description

A while back we had a platform image helper, which would use JavaScript to serve different images based on operating system (e.g. Windows, macOS, Linux). That code no longer exists today, but there have been a couple of requests recently where this kind of functionality would have been useful again.

Reinstating a helper like this today would likely be a little more complex, since we now often serve responsive images and not just 2x high-res versions. We could either create a new, special helper just for this use case, or alternatively update resp_img() with some new functionality. Adding data attributes like data-srcset-win and data-srcset-mac etc, which could be swapped out with srcset using JS might work.