add blue gradient background

in_one_weekend/src/ray.zig

@@ -0,0 +1,11 @@
+const Vec3 = @import("vec3.zig").Vec3;
+const Point3 = @import("vec3.zig").Point3;
+
+pub const Ray = struct {
+    origin: Point3,
+    direction: Vec3,
+
+    pub fn at(t: f32) Point3 {
+        return origin.add(direction.mul(t));
+    }
+};