Revisit Popover caret position and distance from origin

[shawnbot]

This is a follow-on issue for #465, specifically this thread regarding two aspects of the Popover component:

1. The distance of the caret (triangular part) varies between horizontal and vertical orientations. It probably shouldn't.
2. @pifafu suggested that the point at the end of the caret be offset from its "origin" point, possibly somewhere between 2 and 3 spacing units.

While we're in there we should tidy up this mess of SCSS, and possibly incorporate "standard" caret styles as discussed in github/design-systems#406.

[github-actions]

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