fix(icon-button): font awesome icons not centered (#10843)

Co-authored-by: Simeon Simeonoff <[email protected]>

Author: SisIvanova

projects/igniteui-angular/src/lib/core/styles/components/button/_button-theme.scss

@@ -396,6 +396,8 @@
             height: $icon-in-button-size;
             font-size: $icon-in-button-size;
             color: inherit;
+            display: flex;
+            justify-content: center;
         }
 
         @include icon-w-margin(