Always resolvable `Val` alternative with no `Auto` variant

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

Val's Auto variant doesn't make sense for certain Style fields, such as the gap properties. In those cases internally `Auto` just gets mapped to Taffy's LengthPercentage::Points(0.)

This might not seem that significant but it's been bothering me intensely the last year. It's never ideal to have multiple representations mapping to the same underlying value like this.

## What solution would you like?

My proposal is PR #8096 which I submitted a while ago. It needs an update to support the recent changes to `Val`, and the macro that implements some of the helper functions could be improved maybe. There wouldn't be any major changes though, so reviews won't be wasted.

Making `Margin`, `Padding` and `Border` into separate types might be controversial. I'm not bothered if we have a generic `UiRect` or `UiRect` (with `Val` edge values) and `NumRect` (with `Num`s) instead. The only important change is adding the always resolvable type.

## What alternative(s) have you considered?

There are some discussions at: https://github.com/bevyengine/bevy/pull/7656, https://github.com/bevyengine/bevy/pull/7969, https://github.com/bevyengine/bevy/pull/7710, https://github.com/bevyengine/bevy/pull/7569.

Start by reviewing PR #8096 against the current Val changes, then read the discussions in PRs #7656, #7969, #7710, and #7569. Done means the relevant Style fields use an always-resolvable type without an Auto variant, with the chosen Margin, Padding, Border, or UiRect design agreed and implemented.

Written by the indexing model from the issue text.