Manage Zones from the Customizer

Open to a pull request adding Zone management to the Customizer? I started a [new Zoninator](https://github.com/danielbachhuber/collections) under this premise, but decided I don't really want to maintain it.

Start by reviewing the existing Zoninator project and the linked new Zoninator repository to understand the proposed Customizer-based zone management. The issue does not name files, tests, or a concrete acceptance condition, so the work is not considered done until the scope and expected Customizer behavior are defined.

Written by the indexing model from the issue text.