Add now() built-in function

This commit introduces the `now()` built-in function, which returns the
current timestamp in milliseconds. It also includes an integration test
to ensure that `now()` is not constant-folded during AST dumping,
verifying its non-deterministic nature.

src/ast/rtl/datetime.rs

@@ -27,4 +27,14 @@ pub fn register(env: &Environment) {
         }
         Value::Void
     });
+
+    let now_ty = StaticType::Function(Box::new(Signature {
+        params: StaticType::Tuple(vec![]),
+        ret: StaticType::DateTime,
+    }));
+
+    env.register_native("now", now_ty, Purity::SideEffectFree, |_| {
+        let ts = chrono::Utc::now().timestamp_millis();
+        Value::DateTime(ts)
+    });
 }