egui_flex: Ability to omit gap before and/or after an item

[yuriks]

This would be mostly useful when using flex.grow() (or a similarly empty container), to avoid them adding padding when used at the start/end of the container, and to avoid the doubled gap in the middle, as in this screenshot (where I've drawn red markers where the grow()s were added):

I could also see it being handy for other kinds of widgets that want to be visually flush with neighbors.

(I don't know how viable this is to implement. I'm looking into doing it and will PR if I get somewhere, but I'm unfamiliar with the code and details of egui layouting in general.)

[lucasmerlin]

Turns out we have a similar thing with atoms in egui, I'll see what we end up doing there and maybe add a similar solution here: emilk/egui#8026

I don't think something like this exist in css though, so it'd be pretty custom.