[R-package] Make returned feature importances from lgb.importance() visible by default (#4194)

* Return feature importances as visible

Feature importances are returned as invisible. I don't get why would anyone want them invisible. This PR fixes it.

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

Thanks, didn't realize that, changed now.
Co-authored-by: James Lamb <jaylamb20@gmail.com>
Co-authored-by: James Lamb <jaylamb20@gmail.com>

R-package/R/lgb.importance.R

@@ -75,6 +75,7 @@ lgb.importance <- function(model, percentage = TRUE) {
     )]
   }
 
-  return(tree_imp_dt)
+  # adding an empty [] to ensure the table is printed the first time print.data.table() is called
+  return(tree_imp_dt[])
 
 }