Functionality to reference another group of components inside a component schema

[Yoda-Soda]

Currently the component browser does not render/recognise nested array structures.

[bglw]

To enable the following component pattern:

# wrapper.bookshop.toml
[props]
content_blocks = [] # Array of components tagged as the `content_blocks` structure

Ideally, this flow would provide some functionality in the component browser but would leave the field empty for the CMS to use the existing structure definitions. The current implementation will work in a CMS, but will not provide any useful testing environment within the component browser.

[bglw]

Implemented!