build(rust): vendor rnix parser crate

third_party/crates/rnix-0.14.0/test_data/parser/success/docs.nix

@@ -0,0 +1,14 @@
+# Test used by examples/list-fns.rs
+rec {
+  # Usage: add x y
+  # Adds the integers x and y together and returns the result
+  add =
+    # First integer
+    x:
+    # Second integer
+    y:
+    x + y;
+  # Usage: sum nums
+  # Returns the sum of the integer array nums
+  sum = nums: builtins.foldl' add 0 nums;
+}