Add Support for Group block

[pinarol]

Let's add support for Group block to test inner block enhancements. It is the most simple block that uses the InnerBlock component and it has the functionality of nesting in multiple levels.

For this iteration we don't need to worry about adding the block settings.

Let's also not forget to add a check in the block registration(similar to the one for Media & Text block) so that it is not available for production yet, we should be seeing this only on development.