[R-package] deprecate uses of '...' in Dataset slice() method (#4572)

* [R-package] deprecate uses of '...' in Dataset slice() method

* Update R-package/R/lgb.Dataset.R

* Update R-package/R/lgb.Dataset.R
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

R-package/R/lgb.Dataset.R

@@ -500,6 +500,16 @@ Dataset <- R6::R6Class(
     # Slice dataset
     slice = function(idxset, ...) {
 
+      additional_params <- list(...)
+      if (length(additional_params) > 0L) {
+        warning(paste0(
+          "Dataset$slice(): Found the following passed through '...': "
+          , paste(names(additional_params), collapse = ", ")
+          , ". These are ignored and should be removed. "
+          , "In future releases of lightgbm, this warning will become an error."
+        ))
+      }
+
       # Perform slicing
       return(
         Dataset$new(