Auto merge of #96150 - est31:unused_macro_rules, r=petrochenkov

Implement a lint to warn about unused macro rules

This implements a new lint to warn about unused macro rules (arms/matchers), similar to the `unused_macros` lint added by #41907 that warns about entire macros.

```rust
macro_rules! unused_empty {
    (hello) => { println!("Hello, world!") };
    () => { println!("empty") }; //~ ERROR: 1st rule of macro `unused_empty` is never used
}

fn main() {
    unused_empty!(hello);
}
```

Builds upon #96149 and #96156.

Fixes #73576

compiler/rustc_lint/src/lib.rs

@@ -303,6 +303,7 @@ fn register_builtins(store: &mut LintStore, no_interleave_lints: bool) {
         PATH_STATEMENTS,
         UNUSED_ATTRIBUTES,
         UNUSED_MACROS,
+        UNUSED_MACRO_RULES,
         UNUSED_ALLOCATION,
         UNUSED_DOC_COMMENTS,
         UNUSED_EXTERN_CRATES,