mbe: Parse macro attribute rules

This handles various kinds of errors, but does not allow applying the
attributes yet.

This adds the feature gate `macro_attr`.

compiler/rustc_feature/src/unstable.rs

@@ -554,6 +554,8 @@ declare_features! (
     (unstable, link_arg_attribute, "1.76.0", Some(99427)),
     /// Allows fused `loop`/`match` for direct intraprocedural jumps.
     (incomplete, loop_match, "1.90.0", Some(132306)),
+    /// Allow `macro_rules!` attribute rules
+    (unstable, macro_attr, "CURRENT_RUSTC_VERSION", Some(83527)),
     /// Give access to additional metadata about declarative macro meta-variables.
     (unstable, macro_metavar_expr, "1.61.0", Some(83527)),
     /// Provides a way to concatenate identifiers using metavariable expressions.