maintainers: add kmcopper

maintainers/maintainer-list.nix

···
     github = "klntsky";
     githubId = 18447310;
   };
+  kmcopper = {
+    email = "[e-mail]";
+    name = "Kyle Copperfield";
+    github = "kmcopper";
+    githubId = 57132115;
+  };
   kmeakin = {
     email = "[e-mail]";
     name = "Karl Meakin";