fix(drag-drop) : improves drop indicator positioning in draggable context

[geotrev]

Detail

While enhancing the drag and drop pattern with DraggableList.DropIndicator, it became clear it wasn't fully compatible with CSS transform (commonly used in dnd libraries).

This PR does two things:

• Improves DraggableList.DropIndicator's compatibility with CSS transform; the result is significantly less overhead, e.g. spacing offsets and tests
• Enhances the pattern example to align more closely to use-cases Garden expects, including options to test:
  • Horizontal list
  • Drop indicator
  • Placeholder
  • Compact
  • Bare
  • Disabled

The only thing not covered in the pattern overhaul is showing danger state on Dropzone, but that'll be included in website docs.

Checklist

• 👌 design updates will be Garden Designer approved (add the designer as a reviewer) (no change)
• 🌐 demo is up-to-date (yarn start)
• ⬅️ renders as expected with reversed (RTL) direction
• 🤘 renders as expected with Bedrock CSS (?bedrock)
• 💂‍♂️ includes new unit tests. Maintain existing coverage (always >= 96%)
• ♿ tested for WCAG 2.1 AA accessibility compliance (no change)
• 📝 tested in Chrome, Firefox, Safari, and Edge

[jzempel]

A couple of observations on the pattern demo:

• The drop indicator is not available with keyboard (non-mouse) interaction
• In vertical mode, the drop indicator shows at the top of the list by default, but the item goes to the bottom when dropped

I'm not sure if these are working as expected or represent short-comings with dnd-kit. Depending on the outcome, I may have a follow-on comment/concern re: the WAI-ARIA approach with the drop indicator <li>.

[geotrev]

@jzempel I believe it's related to dnd-kit but I'll look into it today. 👌🏻

[jzempel]

Is there a particular treatment that hasPlaceholder is supposed to have on the demo?

[geotrev]

@jzempel hasPlaceholder should show a gray box where a draggable was before the drag started:

(in the above, "Corn" is being dragged)

[coveralls]

Coverage: 96.144% (-0.04%) from 96.179% when pulling 3752f45 on george/drag-drop-patterns into 6733c1d on main.