[UX] Blocks: Add a "Taxonomy term: Depth" Visibility condition

Describe your issue or idea

Just as I often have different layouts for nodes, by node-type, I often need different layouts for categories based on term-depth. For example, my top-level terms need landing pages, but my deeper terms could use their own layout (or even the default layout).

However, without a Visibility condition to select only the top-level terms, I can't do that. Here's one that matches the term depth condition from Panels.

Advocate: @stpaultim