fontconfig: add 'configFile.<name>.target' option

This brings `fonts.fontconfig.configFile` in line with `home.file` and
the like.

tests/modules/misc/fontconfig/extra-config-files.nix

@@ -29,6 +29,10 @@ in
         priority = 37;
         text = "";
       };
+      target = {
+        target = "target";
+        text = "";
+      };
       text.text = sampleText;
       source.source = sampleSource;
     };
@@ -43,6 +47,8 @@ in
 
     assertFileExists ${fcConfD}/37-hm-priority.conf
 
+    assertFileExists ${fcConfD}/target
+
     assertFileExists  ${fcConfD}/90-hm-text.conf
     assertFileContent ${fcConfD}/90-hm-text.conf \
       ${builtins.toFile "sample-text-config" sampleText}