UiRect::AUTO (#18359)

# Objective

Add a `UiRect::AUTO` const which is a `UiRect` with all its edge values
set to `Val::Auto`.

IIRC `UiRect`'s default for its fields a few versions ago was
`Val::Auto` because positions were represented using a `UiRect` and they
required `Val::Auto` as a default. Then when position was split up and
the `UiRect` default was changed, we forgot add a `UiRect::AUTO` const.

Author: ickshonpe

crates/bevy_ui/src/geometry.rs

@@ -349,6 +349,13 @@ impl UiRect {
         bottom: Val::ZERO,
     };
 
+    pub const AUTO: Self = Self {
+        left: Val::Auto,
+        right: Val::Auto,
+        top: Val::Auto,
+        bottom: Val::Auto,
+    };
+
     /// Creates a new [`UiRect`] from the values specified.
     ///
     /// # Example