writers: add writeFish and writeFishBin

This adds writers for Fish shell scripts, with syntax checking.

pkgs/build-support/writers/default.nix

@@ -119,6 +119,21 @@ let
   writeDashBin = name:
     writeDash "/bin/${name}";
 
+  # Like writeScript but the first line is a shebang to fish
+  #
+  # Example:
+  #   writeFish "example" ''
+  #     echo hello world
+  #   ''
+  writeFish = makeScriptWriter {
+    interpreter = "${pkgs.fish}/bin/fish";
+    check = "${pkgs.fish}/bin/fish --no-execute";  # syntax check only
+  };
+
+  # Like writeScriptBin but the first line is a shebang to fish
+  writeFishBin = name:
+    writeFish "/bin/${name}";
+
   # writeHaskell takes a name, an attrset with libraries and haskell version (both optional)
   # and some haskell source code and returns an executable.
   #

pkgs/build-support/writers/test.nix

@@ -22,6 +22,12 @@ let
      test '~' = '~' && echo 'success'
     '';
 
+    fish = writeFishBin "test-writers-fish-bin" ''
+      if test "test" = "test"
+        echo "success"
+      end
+    '';
+
     rust = writeRustBin "test-writers-rust-bin" {} ''
       fn main(){
         println!("success")
@@ -94,6 +100,12 @@ let
      test '~' = '~' && echo 'success'
     '';
 
+    fish = writeFish "test-writers-fish" ''
+      if test "test" = "test"
+        echo "success"
+      end
+    '';
+
     haskell = writeHaskell "test-writers-haskell" { libraries = [ haskellPackages.acme-default ]; } ''
       import Data.Default