move `BorderRect` into `bevy_math`

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

It's in `bevy_sprite::texture_slice` at the moment. I've been using it in the UI to hold border and padding values though.

## What solution would you like?

Move it into `bevy_math::rects`.

## What alternative(s) have you considered?

We could just use `Rect` to hold inset/border/padding values. Different types make its easier to understand that it represents inset values relative to a rectangle and not a rectangle itself.