maintainers/matrix: add transcaffeine

She's been tirelessly keeping Synapse up-to-date and also worked quite a
lot on the Element packages, so having her officially part of this makes
sense. I confirmed with her in private that she's interested.

maintainers/team-list.nix

@@ -510,6 +510,7 @@ with lib.maintainers;
       dandellion
       nickcao
       teutat3s
+      transcaffeine
     ];
     scope = "Maintain the ecosystem around Matrix, a decentralized messenger.";
     shortName = "Matrix";