Change bevy_reflect::RegisterForReflection::__register() to expect unused variables, rather than putting underscores on the parameter names (#17171)

# Objective
While checking over https://github.com/bevyengine/bevy/pull/17160, it
occurred to me that rust-analyzer will copy the method signature
exactly, when using tab completion trait methods. This includes provided
trait methods that use underscores to silence the `unused_variables`
lint. This probably isn't good for users, seeing as how they'll have to
remove the underscore if they want to use the parameters.

(I mean, they technically don't have to remove the underscore... but
usually you don't keep a leading underscore on parameters you're using.)

## Solution
Changes `bevy_reflect::RegisterForReflection::__register()` to
`#[expect(unused_variables)]`, and removes the underscores from its
parameter names.

## Testing
N/A

crates/bevy_reflect/src/lib.rs

@@ -691,7 +691,11 @@ pub mod __macro_exports {
         note = "consider annotating `{Self}` with `#[derive(Reflect)]`"
     )]
     pub trait RegisterForReflection {
-        fn __register(_registry: &mut TypeRegistry) {}
+        #[expect(
+            unused_variables,
+            reason = "The parameters here are intentionally unused by the default implementation; however, putting underscores here will result in the underscores being copied by rust-analyzer's tab completion."
+        )]
+        fn __register(registry: &mut TypeRegistry) {}
     }
 
     impl<T: GetTypeRegistration> RegisterForReflection for T {