Improve look and consistency of GlobalDrawerActionItem
This commit makes the following visual changes to GlobalDrawerActionItem list items:
- Turns on separators between list items, to match the appearance of other list items in sidebars and drawers
- Reduces the size and opacity of the arrow on the right side for un-collapsed list items to make it look nicer and de-emphasize it visually
| Discover, before | Discover, After |
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| Kirigami Gallery, before | Kirigami Gallery, After |
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cc @teams/vdg