Implement built in functions opacity and alpha

src/builtin/color.rs

@@ -39,4 +39,16 @@ pub(crate) fn register(f: &mut BTreeMap<String, Builtin>) {
             _ => todo!("non-color given to builtin function `blue()`")
         }
     });
+    decl!(f "opacity", |args, _| {
+        match arg!(args, 0, "color") {
+            Value::Color(c) => Some(Value::Dimension(BigRational::from_integer(BigInt::from(c.alpha())), Unit::None)),
+            _ => todo!("non-color given to builtin function `opacity()`")
+        }
+    });
+    decl!(f "alpha", |args, _| {
+        match arg!(args, 0, "color") {
+            Value::Color(c) => Some(Value::Dimension(BigRational::from_integer(BigInt::from(c.alpha())), Unit::None)),
+            _ => todo!("non-color given to builtin function `alpha()`")
+        }
+    });
 }

src/color/mod.rs

@@ -36,6 +36,10 @@ impl Color {
         self.green
     }
 
+    pub const fn alpha(&self) -> u16 {
+        self.alpha
+    }
+
     pub fn from_values(red: u16, green: u16, blue: u16, alpha: u16) -> Self {
         let repr = if alpha >= 1 {
             format!("#{:0>2x}{:0>2x}{:0>2x}", red, green, blue)