add Lambertian material

in_one_weekend/src/hittable.zig

@@ -1,12 +1,14 @@
 const Point3 = @import("vec3.zig").Point3;
 const Vec3 = @import("vec3.zig").Vec3;
 const Ray = @import("ray.zig").Ray;
+const Material = @import("material.zig").Material;
 
 pub const HitRecord = struct {
     p: Point3,
     normal: Vec3,
     t: f32,
     front_face: bool,
+    material: Material,
 
     pub fn setFaceNormal(hit: *HitRecord, ray: Ray, outward_normal: Vec3) void {
         const front_face = Vec3.dot(ray.direction, outward_normal) < 0;