feat(nix): some highlight improvements

- functions
- builtin functions with prefix
- add test file

tests/query/highlights/nix/test.nix

@@ -0,0 +1,16 @@
+{
+  func1 = param: builtins.readFile param;
+  # ^ @function
+        # ^ @parameter
+                # ^ @constant.builtin
+                         # ^ @function.builtin
+  func2 = { p1, p2 }: p2;
+  # ^ @function
+          # ^ @parameter
+  readFile' = readFile;
+              # ^ @function.builtin
+  x = func1 ./path/to/file.nix;
+# ^ @field
+      # ^ @function.call
+              # ^ @string.special
+}