style(grid): position the add row snackbar central

Author: SisIvanova

projects/igniteui-angular/src/lib/core/styles/components/grid/_grid-theme.scss

@@ -2338,8 +2338,9 @@
     %igx-grid__addrow-snackbar {
         position: absolute;
         z-index: 5;
-        left: 35%;
         bottom: 25%;
+        left: 50%;
+        transform: translateX(-50%);
     }
 
     %igx-grid__filtering-cell {