illuminants: Add D50

This is another common illuminant.
diff --git a/src/commonMain/kotlin/dev/kdrag0n/colorkt/Illuminants.kt b/src/commonMain/kotlin/dev/kdrag0n/colorkt/Illuminants.kt
index c9a440f..c3b65fd 100644
--- a/src/commonMain/kotlin/dev/kdrag0n/colorkt/Illuminants.kt
+++ b/src/commonMain/kotlin/dev/kdrag0n/colorkt/Illuminants.kt
@@ -8,4 +8,10 @@
         y = 1.0,
         z = 1.08883,
     )
+
+    val D50 = CieXyz(
+        x = 0.9642,
+        y = 1.0,
+        z = 0.8251,
+    )
 }