Add support for selecting a layout

[mogmarsh]

Description

When the user adds a Query block, there should be a prompt with several predefined layouts - like the core Query Loop block does. These should be modifiable by filter or adding files to a certain location (or both).

Acceptance Criteria

• Add predefined layouts that mirror what the core Query Loop block does (use core's functionality to the extent possible here).
• Allow the layouts to be filterable, ideally in PHP, so theme authors can add their own.