maintainers: Add istoph and textshell

I did this on their behalf, they'll approve the PR.

maintainers/maintainer-list.nix

···
     githubId = 13622947;
     keys = [ { fingerprint = "1412 816B A9FA F62F D051 1975 D3E1 B013 B463 1293"; } ];
   };
+  istoph = {
+    email = "[e-mail]";
+    name = "Christoph Hüffelmann";
+    github = "istoph";
+    githubId = 114227790;
+  };
   ius = {
     email = "[e-mail]";
     name = "Joerie de Gram";
···
     github = "tex";
     githubId = 27386;
     name = "Milan Svoboda";
+  };
+  textshell = {
+    email = "[e-mail]";
+    github = "textshell";
+    githubId = 6579711;
+    name = "Martin Hostettler";
   };
   tfc = {
     email = "[e-mail]";