[2.0.0-beta4] Fix component stories overrides

Problem/Motivation

When we call a component story and we override some slots and/or props value:

type: component
component: 'ui_suite_bootstrap:button'
story: preview
 slots:
   label: Test
props:
  variant: success

The override doesn't work

Proposed resolution

Reverse the merges:

-    $slots = array_merge($element["#slots"] ?? [], $story["slots"] ?? []);
+    $slots = array_merge($story["slots"] ?? [], $element["#slots"] ?? []);

-    $element["#props"] = array_merge($element["#props"] ?? [], $story["props"] ?? []);
+    $element["#props"] = array_merge($story["props"] ?? [], $element["#props"] ?? []);