lib/strings: add concatLines

Like `unlines` from Haskell.

The aim is to replace the `concatStringsSep "\n"` pattern for generated
files, which doesn't add a final newline.

lib/tests/misc.nix

@@ -153,6 +153,11 @@ runTests {
     expected = "a,b,c";
   };
 
+  testConcatLines = {
+    expr = concatLines ["a" "b" "c"];
+    expected = "a\nb\nc\n";
+  };
+
   testSplitStringsSimple = {
     expr = strings.splitString "." "a.b.c.d";
     expected = [ "a" "b" "c" "d" ];