maintainers: add matthiasbenaets

maintainers/maintainer-list.nix

···
     githubId = 19036;
     name = "Matthew Bauer";
   };
+  matthiasbenaets = {
+    email = "[e-mail]";
+    github = "MatthiasBenaets";
+    githubId = 89214559;
+    name = "Matthias Benaets";
+  };
   matthiasbeyer = {
     email = "[e-mail]";
     matrix = "@musicmatze:beyermatthi.as";