Alpha/RPCs: rename field labels in Michelson typechecker output

This is a change in field names in the Michelson typechecking RPC output.
A simple solution for third party tools to handle the change is to
recognise both field names during the protocol update period.

src/proto_alpha/lib_protocol/src/script_tc_errors_registration.ml

@@ -37,8 +37,8 @@ let type_map_enc =
        (fun (loc, bef, aft) -> (loc, (bef, aft)))
        (obj3
           (req "location" Script.location_encoding)
-          (req "stackBefore" stack_enc)
-          (req "stackAfter" stack_enc)))
+          (req "stack_before" stack_enc)
+          (req "stack_after" stack_enc)))
 
 let stack_ty_enc =
   let open Data_encoding in