Added StableInterpolate implementations for linear colors. (#18601)

# Objective

Colors currently do not implement `StableInterpolate`, which makes them
ineligible for functions like `smooth_nudge` and make some generic APIs
awkward.

## Solution

Implemented `StableInterpolate` for linear color types that should be
uncontroversial. Non-linear types like `Hsl` are not implemented in this
PR.

## Testing

Added a test that checks implementations are correct.

Author: mintlu8

crates/bevy_color/src/interpolate.rs

@@ -0,0 +1,37 @@
+//! TODO: Implement for non-linear colors.
+
+#[cfg(test)]
+mod test {
+    use bevy_math::StableInterpolate;
+
+    use crate::{Gray, Laba, LinearRgba, Oklaba, Srgba, Xyza};
+
+    #[test]
+    pub fn test_color_stable_interpolate() {
+        let b = Srgba::BLACK;
+        let w = Srgba::WHITE;
+        assert_eq!(
+            b.interpolate_stable(&w, 0.5),
+            Srgba::new(0.5, 0.5, 0.5, 1.0),
+        );
+
+        let b = LinearRgba::BLACK;
+        let w = LinearRgba::WHITE;
+        assert_eq!(
+            b.interpolate_stable(&w, 0.5),
+            LinearRgba::new(0.5, 0.5, 0.5, 1.0),
+        );
+
+        let b = Xyza::BLACK;
+        let w = Xyza::WHITE;
+        assert_eq!(b.interpolate_stable(&w, 0.5), Xyza::gray(0.5),);
+
+        let b = Laba::BLACK;
+        let w = Laba::WHITE;
+        assert_eq!(b.interpolate_stable(&w, 0.5), Laba::new(0.5, 0.0, 0.0, 1.0),);
+
+        let b = Oklaba::BLACK;
+        let w = Oklaba::WHITE;
+        assert_eq!(b.interpolate_stable(&w, 0.5), Oklaba::gray(0.5),);
+    }
+}

crates/bevy_color/src/lib.rs

@@ -105,6 +105,7 @@ mod color_range;
 mod hsla;
 mod hsva;
 mod hwba;
+mod interpolate;
 mod laba;
 mod lcha;
 mod linear_rgba;
@@ -265,6 +266,12 @@ macro_rules! impl_componentwise_vector_space {
                 $($element: 0.0,)+
             };
         }
+
+        impl bevy_math::StableInterpolate for $ty {
+            fn interpolate_stable(&self, other: &Self, t: f32) -> Self {
+                bevy_math::VectorSpace::lerp(*self, *other, t)
+            }
+        }
     };
 }