Form block: Conditional field display

### What

Form fields should have a conditional display option.

"If this field is checked then display these fields."

### Why

This would add flexibility to the Form block. Having fields hidden behind conditional display logic would allow the creation of smaller more condensed forms. This is a common feature with form builders.

Example uses:



### How

The fields could have a setting for this, [this plugin](https://wordpress.org/plugins/cf7-conditional-fields/) shows this feature for another form solution.

The issue names the Form block but no repository file or test. Start by locating the Form block entry point and compare the linked conditional-fields example; define the supported field relationships and their editor and front-end behavior before implementation. Done means conditional fields can be configured and reliably shown or hidden in a form.

Written by the indexing model from the issue text.