chore: use cfg! macro instead of #[cfg] attribute (#3446)

this unifies the code path. The alternative will *move* arguments passed
to the debug_warn! macro only when compiling without debug_assertions.
This version will instead always use additional format arguments by reference.

leptos_dom/src/logging.rs

@@ -29,14 +29,9 @@ macro_rules! error {
 macro_rules! debug_warn {
     ($($x:tt)*) => {
         {
-            #[cfg(debug_assertions)]
-            {
+            if cfg!(debug_assertions) {
                 $crate::warn!($($x)*)
             }
-            #[cfg(not(debug_assertions))]
-            {
-                ($($x)*)
-            }
         }
     }
 }