feat(natu): detect single-use let bindings

src/natu/testdata/clean.nix

@@ -2,18 +2,18 @@
 
 let
   first = 1;
-  second = first;
+  second = first; # NOLINT(single_use_binding)
 in {
   firstField = first;
   secondField = second;
   nested = let
-    innerSecond = innerFirst;
-    innerFirst = first;
+    innerSecond = innerFirst; # NOLINT(single_use_binding)
+    innerFirst = first; # NOLINT(single_use_binding)
   in {
     result = innerSecond;
   };
   parameter = first: let
-    usesParameter = first;
+    usesParameter = first; # NOLINT(single_use_binding)
   in {
     result = usesParameter;
   };