Change Urect::width & Urect::height to be const (#9640)

# Objective
The two functions

[`Urect::height`](https://docs.rs/bevy_math/latest/bevy_math/struct.URect.html#method.height),

[`Urect::width`](https://docs.rs/bevy_math/latest/bevy_math/struct.URect.html#method.width)
 are currently not const. 
Since the methods are very unlikely to change (ever) and are useful to
be const for some games, they should be.

Co-authored-by: Emi <emanuel.boehm@gmail.com>

crates/bevy_math/src/rects/urect.rs

@@ -130,7 +130,7 @@ impl URect {
     /// assert_eq!(r.width(), 5);
     /// ```
     #[inline]
-    pub fn width(&self) -> u32 {
+    pub const fn width(&self) -> u32 {
         self.max.x - self.min.x
     }
 
@@ -144,7 +144,7 @@ impl URect {
     /// assert_eq!(r.height(), 1);
     /// ```
     #[inline]
-    pub fn height(&self) -> u32 {
+    pub const fn height(&self) -> u32 {
         self.max.y - self.min.y
     }