nixos/programs/tmux: specify wanted plugins

Currently it's rather difficult to install tmux plugins. The process involves two steps:
  1. Specify the correct `pkg.tmuxPlugins` package in `environment.systemPackages`
  2. Adding to the configuration file to instantiate the plugin.

This commit allows the user to specify a list of plugins under `programs.tmux.plugins`.

Update nixos/modules/programs/tmux.nix

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

nixos/doc/manual/from_md/release-notes/rl-2205.section.xml

@@ -712,6 +712,15 @@
           warning.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          <literal>programs.tmux</literal> has a new option
+          <literal>plugins</literal> that accepts a list of packages
+          from the <literal>tmuxPlugins</literal> group. The specified
+          packages are added to the system and loaded by
+          <literal>tmux</literal>.
+        </para>
+      </listitem>
     </itemizedlist>
   </section>
 </section>

nixos/doc/manual/release-notes/rl-2205.section.md

@@ -243,4 +243,6 @@ In addition to numerous new and upgraded packages, this release has the followin
   Reason is that the old name has been deprecated upstream.
   Using the old option name will still work, but produce a warning.
 
+- `programs.tmux` has a new option `plugins` that accepts a list of packages from the `tmuxPlugins` group. The specified packages are added to the system and loaded by `tmux`.
+
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->

nixos/modules/programs/tmux.nix

@@ -52,6 +52,12 @@ let
     set  -s escape-time       ${toString cfg.escapeTime}
     set  -g history-limit     ${toString cfg.historyLimit}
 
+    ${lib.optionalString (cfg.plugins != []) ''
+    # Run plugins
+    ${lib.concatMapStringsSep "\n" (x: "run-shell ${x.rtp}") cfg.plugins}
+
+    ''}
+
     ${cfg.extraConfig}
   '';
 
@@ -165,6 +171,13 @@ in {
           downside it doesn't survive user logout.
         '';
       };
+
+      plugins = mkOption {
+        default = [];
+        type = types.listOf types.package;
+        description = "List of plugins to install.";
+        example = lib.literalExpression "[ pkgs.tmuxPlugins.nord ]";
+      };
     };
   };
 
@@ -174,7 +187,7 @@ in {
     environment = {
       etc."tmux.conf".text = tmuxConf;
 
-      systemPackages = [ pkgs.tmux ];
+      systemPackages = [ pkgs.tmux ] ++ cfg.plugins;
 
       variables = {
         TMUX_TMPDIR = lib.optional cfg.secureSocket ''''${XDG_RUNTIME_DIR:-"/run/user/$(id -u)"}'';