bevy_ui: Implement Val::ObjectFit for width and height fields on bevy_ui::ui_node::Style

[vveisard]

What problem does this solve or what need does it fill?

It's awkward to size and position multiple child UI elements without this property.

What solution would you like?

Implement the behavior of this CSS property for width and height fields of Style. https://developer.mozilla.org/en-US/docs/Web/CSS/object-fit

The solution you propose for the problem presented.

What alternative(s) have you considered?

Other solutions to solve and/or work around the problem presented.