Navigation: Refine and polish the dropdown menus

[jasmussen]

From WordPress/wporg-main-2022#221

Dropdown shall have some bottom padding in the menu – when the last item is active it looks visually stressed.
The dropdown flyout is 1px off to the left.

• Add some bottom padding to the dropdown menu

Mockup that adds 12px extra bottom padding:

Note that this will be less urgent if #360 is addressed, but would still be worth having.

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Secondly, there's an issue with the dropdown positioning, it's 1px off, which is most visible on the About subpages:

• Fix the positioning of the dropdown menu to not be 1px off

This is likely due to the left: -1px; rule applied to the dropdown menu:

This negative margin exists when the menu uses "default colors" (in this case external colors unknown to the navigation block), and this is set to 0 as soon as the navigation block has-background, for example. See also: https://github.com/WordPress/gutenberg/blob/trunk/packages/block-library/src/navigation/style.scss#L272