Implement builtin function grayscale()

src/builtin/color.rs

@@ -314,4 +314,11 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
         };
         Some(Value::Color(color.fade_out(amount)))
     });
+    decl!(f "grayscale", |args, _| {
+        let color = match arg!(args, 0, "color").eval() {
+            Value::Color(c) => c,
+            _ => todo!("non-color given to builtin function `grayscale()`")
+        };
+        Some(Value::Color(color.desaturate(Number::from(1))))
+    });
 }

tests/color.rs

@@ -341,3 +341,13 @@ test!(
     "a {\n  color: opacify(rgba(0, 0, 17, 0.8), 0.2);\n}\n",
     "a {\n  color: #000011;\n}\n"
 );
+test!(
+    grayscale_1,
+    "a {\n  color: grayscale(plum);\n}\n",
+    "a {\n  color: #bfbfbf;\n}\n"
+);
+test!(
+    grayscale_2,
+    "a {\n  color: grayscale(red);\n}\n",
+    "a {\n  color: gray;\n}\n"
+);