fish: format user and generated .fish files

Adds a `fishIndent` wrapper to pass fish scripts to the built in
`fish_indent` function.

tests/modules/programs/fish/format-scripts.nix

@@ -0,0 +1,50 @@
+{ config, pkgs, ... }:
+
+let
+
+  expectedFunc = pkgs.writeText "func.fish" ''
+    function func
+        echo foo
+    end
+  '';
+
+  expectedFuncMulti = pkgs.writeText "func-multi.fish" ''
+    function func-multi
+        echo bar
+        if foo
+            bar
+            baz
+        end
+    end
+  '';
+
+in {
+  config = {
+    programs.fish = {
+      enable = true;
+
+      formatFishScripts = true;
+
+      functions = {
+        func = ''echo "foo"'';
+        func-multi = ''
+              echo bar
+          if foo
+              bar
+                  baz
+                end
+        '';
+      };
+    };
+
+    nmt.script = ''
+      assertFileExists home-files/.config/fish/functions/func.fish
+      echo ${expectedFunc}
+      assertFileContent home-files/.config/fish/functions/func.fish ${expectedFunc}
+
+      assertFileExists home-files/.config/fish/functions/func-multi.fish
+      echo ${expectedFuncMulti}
+      assertFileContent home-files/.config/fish/functions/func-multi.fish ${expectedFuncMulti}
+    '';
+  };
+}