feat(tmux): add a native Module and enable it on neogaia

Configure tmux through home-manager's programs.tmux: the settings it exposes
as options (prefix, keyMode, mouse, baseIndex, clock24, escapeTime,
historyLimit, terminal) are set as options, and every setting it has no option
for is read verbatim from modules/tmux/extra.conf. No tmux plugin manager is
used. The generated config is behaviourally identical to the reference, verified
by parsing it with a live tmux binary.

hosts/neogaia/default.nix

@@ -39,6 +39,9 @@
   modules.fish.enable = true;
   modules.fish.defaultShell = true;
 
+  # tmux as the terminal multiplexer.
+  modules.tmux.enable = true;
+
   # Locale preferences for the base system.
   time.timeZone = "America/New_York";
   i18n.defaultLocale = "en_GB.UTF-8";