Allow unreachable code in #[debug_handler] (#2014)

Co-authored-by: David Pedersen <[email protected]>

axum-macros/tests/debug_handler/pass/deny_unreachable_code.rs

@@ -0,0 +1,8 @@
+#![deny(unreachable_code)]
+
+use axum::extract::Path;
+
+#[axum_macros::debug_handler]
+async fn handler(Path(_): Path<String>) {}
+
+fn main() {}