servoshell: allow event tracing to be configured with RUST_LOG (#31657)

* servoshell: allow event tracing to be configured with RUST_LOG

* Move docs to same file as target prefixes

* Avoid requiring macro callers to use LogTarget trait

* Clean up macro call in app.rs

* Rename macros to reflect value types

ports/servoshell/lib.rs

@@ -9,6 +9,9 @@
 #[macro_use]
 extern crate sig;
 
+#[macro_use]
+mod tracing;
+
 #[cfg(test)]
 mod test;