wrapFish: add fish shell wrapper package

This adds a wrapper for fish which allows creating shells pre-initialised
with some completions, functions, and configuration scripts from given paths
or from fish plugin packages (`pkgs.fishPlugins.*`).

This is especially handy when one wants to try a plugin in an ephemeral shell.

GitHub: see https://github.com/NixOS/nixpkgs/pull/107834#discussion_r550612519

pkgs/shells/fish/wrapper.nix

@@ -0,0 +1,25 @@
+{ lib, writeShellScriptBin, fish }:
+
+with lib;
+
+makeOverridable ({
+  completionDirs ? [],
+  functionDirs ? [],
+  confDirs ? [],
+  pluginPkgs ? []
+}:
+
+let
+  vendorDir = kind: plugin: "${plugin}/share/fish/vendor_${kind}.d";
+  complPath = completionDirs ++ map (vendorDir "completions") pluginPkgs;
+  funcPath = functionDirs ++ map (vendorDir "functions") pluginPkgs;
+  confPath = confDirs ++ map (vendorDir "conf") pluginPkgs;
+  safeConfPath = map escapeShellArg confPath;
+
+in writeShellScriptBin "fish" ''
+  ${fish}/bin/fish --init-command "
+    set --prepend fish_complete_path ${escapeShellArgs complPath}
+    set --prepend fish_function_path ${escapeShellArgs funcPath}
+    for c in {${concatStringsSep "," safeConfPath}}/*; source $c; end
+  " "$@"
+'')

pkgs/top-level/all-packages.nix

@@ -8843,6 +8843,8 @@ in
 
   fish = callPackage ../shells/fish { };
 
+  wrapFish = callPackage ../shells/fish/wrapper.nix { };
+
   fishPlugins = recurseIntoAttrs (callPackage ../shells/fish/plugins { });
 
   ion = callPackage ../shells/ion {