* Added `Val::ZERO` as a constant which is defined as `Val::Px(0.)`.
* Added manual `PartialEq` implementation for `Val` which allows any zero value to equal any other zero value. E.g., `Val::Px(0.) == Val::Percent(0.)` etc.
* Replaced instances of `Val::Px(0.)`, `Val::Percent(0.)`, etc. with `Val::ZERO`
* Fixed `bevy_ui::layout::convert::tests::test_convert_from` test to account for Taffy not equating `Points(0.)` and `Percent(0.)`. These tests now use `assert_eq!(...)` instead of `assert!(matches!(...))` which gives easier to diagnose error messages.