Conditional / flexible content field support

A _flexible content field_ (similar to the similarly named field in Advanced Custom Fields for WordPress) would add a massive amount of power and functionality to Grav forms.

The idea for this was posted a [Grav discourse post,](https://discourse.getgrav.org/t/conditional-field-creating-a-flexible-content-field-like-advanced-custom-fields-wordpress-plugin/7293) along with some code to essentially get it working. But if this could be integrated into the Form plugin core that would be amazing!

Start with the linked Grav discourse post and its example code, then inspect the Grav Form plugin core to understand how fields are currently defined and processed. The issue does not name specific files, tests, or acceptance criteria; done would mean a flexible content field is integrated into the core with behavior matching the proposed concept.

Written by the indexing model from the issue text.