chore: default resizer block gap to 9 on each sizde

Author: NathanZlion

src/table/resizer/styles.scss

@@ -18,7 +18,7 @@ $active-separator-width: 2px;
 // $block-gap: calc(2 * #{awsui.$space-xs} + #{awsui.$space-xxxs});
 // Use a CSS custom property so the gap can be overridden at runtime (demo/playground for designers).
 // stylelint-disable-next-line custom-property-pattern
-$block-gap: var(--awsui-table-resizer-block-gap, 0px);
+$block-gap: var(--awsui-table-resizer-block-gap, 18px);
 
 .resizer-wrapper {
   inset-block: 0;