Improve error diagnostics

Author: saki7

include/iris/x4/directive/lexeme.hpp

@@ -14,6 +14,7 @@
 #include <iris/x4/core/skip_over.hpp>
 #include <iris/x4/core/parser.hpp>
 
+#include <format>
 #include <iterator>
 #include <type_traits>
 #include <utility>
@@ -43,6 +44,11 @@ struct lexeme_directive : proxy_parser<Subject, lexeme_directive<Subject>>
             attr
         );
     }
+
+    [[nodiscard]] constexpr std::string get_x4_info() const
+    {
+        return std::format("lexeme[{}]", get_info<Subject>{}(this->subject));
+    }
 };
 
 namespace detail {