[WIP] Fix/issue 4437 builder component marker

[techfg]

What does this PR do?

Places an outline on all top-level components in the builder canvas so that they can be identified visually even they do not render anything within.

Currently a WIP to solicit input/feedback before finalizing.

A few thoughts:

1. This is just an initial concept to solicit feedback on the best way to achieve the desired outcome per bug: unable to identify/select component that has no content in builder canvas #4437
2. I chose a min-height: 1.25rem because when a component is selected, it is given an outline of 8px width which equates to 1rem so choosing 1.25rem provides a small amount of whitespace between the red "selected" outline
3. If a component itself returns null, the "empty" classes ensure the contents element itself has a visual border (it uses display:contents so it doesn't have block dimensions itself)
4. I used the same border style (dashed) and color (slate-300) as we use elsewhere so possibly this should be a different style and/or color
5. I wrote the classes such that they can be easily rendered out planning ahead for the new "Action Bar" in the builder that could contain a "Hide Markers" button. The marker itself could be rendered out conditionally in one of two ways:
   1. The button in the action bar would trigger different conditional behavior within buildwrapper.tsx
   2. The button could put a class on the top level element of the builder and then the marker classes could be prefixed with :not(.hide-marker) ...
6. One additional thing to consider is whether or not each slot within a component should be given the same/similar type of marker so that its easy to visually identify each slot vs just the entire component. Additionally, currently, there is no way to drag/drop a component in a slot on the canvas if it doesn't render anything (e.g., card footer), the only way to put something in a slot is via component index or yaml - thoughts?
   • EDIT Actually, it doesn't look like drag-n-drop works within a slot even if contains components that are visible on the canvas. For example, if a Card has 5 text components in the Content slot, I can't drag another one to the Content slot in the canvas, seems only way is to add via component index or yaml. Do slots support drag-n-drop currently?

Testing

Tested in the builder UI with components that render something visual, doesn't render anything visual but has an element itself, renders null/nothing.

Resolves #4437