Site Breadcrumbs should collapse on mobile

[ryelle]

We've solved the breadcrumbs wrapping on mobile in a few sites so far, WordPress/wporg-developer#166, WordPress/wporg-documentation-2022#19, and now WordPress/wporg-showcase-2022#96.

Let's DRY this up and set some better defaults for the block's CSS.

[ryelle]

Came across this nesting while testing something else. We shouldn't assume any fixed number of nested items or that we can hide items at specific breakpoints, because it might need to start wrapping at 1550px wide 😱

For reference, on the current developer site, this wraps:

[adamwoodnz]

Just noting a new community plugin in this space https://gutenbergtimes.com/introducing-the-breadcrumbs-wordpress-block-plugin/

[StevenDufresne]

Since the breadcrumbs will be pulled into the content area and no longer compete with a variable length menu on the right, I don't think this is needed anymore, at least not in the way this ticket was imagining.

[adamwoodnz]

Temporarily reverted #437