Add constants for Direction2d and Direction3d (#11180)

# Objective

I often need a direction along one of the cartesian XYZ axes, and it
currently requires e.g. `Direction2d::from_normalized(Vec2::X)`, which
isn't ideal.

## Solution

Add direction constants that are the same as the ones on Glam types. I
also copied the doc comment format "A unit vector pointing along the ...
axis", but I can change it if there's a better wording for directions.

crates/bevy_math/src/primitives/dim2.rs

@@ -7,6 +7,15 @@ use crate::Vec2;
 pub struct Direction2d(Vec2);
 
 impl Direction2d {
+    /// A unit vector pointing along the positive X axis.
+    pub const X: Self = Self(Vec2::X);
+    /// A unit vector pointing along the positive Y axis.
+    pub const Y: Self = Self(Vec2::Y);
+    /// A unit vector pointing along the negative X axis.
+    pub const NEG_X: Self = Self(Vec2::NEG_X);
+    /// A unit vector pointing along the negative Y axis.
+    pub const NEG_Y: Self = Self(Vec2::NEG_Y);
+
     /// Create a direction from a finite, nonzero [`Vec2`].
     ///
     /// Returns [`Err(InvalidDirectionError)`](InvalidDirectionError) if the length
@@ -378,10 +387,7 @@ mod tests {
 
     #[test]
     fn direction_creation() {
-        assert_eq!(
+        assert_eq!(Direction2d::new(Vec2::X * 12.5), Ok(Direction2d::X));
-            Direction2d::new(Vec2::X * 12.5),
-            Ok(Direction2d::from_normalized(Vec2::X))
-        );
         assert_eq!(
             Direction2d::new(Vec2::new(0.0, 0.0)),
             Err(InvalidDirectionError::Zero)

crates/bevy_math/src/primitives/dim3.rs

@@ -7,6 +7,19 @@ use crate::Vec3;
 pub struct Direction3d(Vec3);
 
 impl Direction3d {
+    /// A unit vector pointing along the positive X axis.
+    pub const X: Self = Self(Vec3::X);
+    /// A unit vector pointing along the positive Y axis.
+    pub const Y: Self = Self(Vec3::Y);
+    /// A unit vector pointing along the positive Z axis.
+    pub const Z: Self = Self(Vec3::Z);
+    /// A unit vector pointing along the negative X axis.
+    pub const NEG_X: Self = Self(Vec3::NEG_X);
+    /// A unit vector pointing along the negative Y axis.
+    pub const NEG_Y: Self = Self(Vec3::NEG_Y);
+    /// A unit vector pointing along the negative Z axis.
+    pub const NEG_Z: Self = Self(Vec3::NEG_Z);
+
     /// Create a direction from a finite, nonzero [`Vec3`].
     ///
     /// Returns [`Err(InvalidDirectionError)`](InvalidDirectionError) if the length
@@ -391,10 +404,7 @@ mod test {
 
     #[test]
     fn direction_creation() {
-        assert_eq!(
+        assert_eq!(Direction3d::new(Vec3::X * 12.5), Ok(Direction3d::X));
-            Direction3d::new(Vec3::X * 12.5),
-            Ok(Direction3d::from_normalized(Vec3::X))
-        );
         assert_eq!(
             Direction3d::new(Vec3::new(0.0, 0.0, 0.0)),
             Err(InvalidDirectionError::Zero)