Fix: values being moved with the debug_warn! macro when compiling with not(debug_assertions)

[WorldSEnder]

This unifies the code path by using the cfg! macro instead of #[cfg] attribute. 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.

A minimal test code the shows the avoided bug:

let foo = "foo".to_string(); // A value that is not copy
leptos::logging::debug_warn!("Message: {}", foo); // foo is passed as an additional argument
let _ = foo.len(); // A usage after logging

The above will complain about foo being moved in the unpatched code only when compiling with not(debug_assertions), since in that case the macro expands to a tuple of the arguments which counts as a usage and moves foo (into the void, but still).

Further advantage of the patched code is that the usual format_args! checks run with and without debug_assertions enable.