maintainers: Add ursi

maintainers/maintainer-list.nix

···
     githubId = 32751441;
     name = "urlordjames";
   };
+  ursi = {
+    email = "[e-mail]";
+    github = "ursi";
+    githubId = 17836748;
+    name = "Mason Mackaman";
+  };
   uskudnik = {
     email = "[e-mail]";
     github = "uskudnik";