[Post Featured Image]: Allow cropping and/or constraining to a certain ratio

[kjellr]

Similar to #33789.

Alongside the featured image resizing controls, we should include the option to crop the featured image down to a specific image ratio. This could be a freeform crop control, or it could be just a set of options (1:1, 4:3, 16:9, etc.).

The user can of course enter absolute pixel sizes to do this sort of thing, but then the image won't scale responsively.

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Example of all square images:

[mtias]

An aspect-ratio control would make sense.

[scruffian]

This is now possible, though we still need to add the aspect ratio controls. Let's close this in favour of #38990