Figured out why dropLast call was producing an argument mismatch:

- Although list_drop_last's LowLevel operation modeling (body) was correct, the defn() CALLED that body with an extra argument for index (a copy-paste error from dropAt).
- List.dropAt works now :)

compiler/can/src/builtins.rs

@@ -2043,7 +2043,7 @@ fn list_drop_last(symbol: Symbol, var_store: &mut VarStore) -> Def {
 
     defn(
         symbol,
-        vec![(list_var, Symbol::ARG_1), (index_var, Symbol::ARG_2)],
+        vec![(list_var, Symbol::ARG_1)],
         var_store,
         body,
         list_var,