refactor(tmux): move tmux into its own module

modules/tmux.nix

@@ -0,0 +1,10 @@
+{ ... }:
+{
+  programs.tmux = {
+    enable = true;
+    extraConfig = ''
+      set -g default-terminal "tmux-256color"
+      set -ag terminal-overrides ",xterm-256color:RGB"
+    '';
+  };
+}