feat: support conditional prompting for variables

**Description**

As a mason user, I would sometimes like to prompt for variables conditionally depending on the previous answers.

Conditional prompting can be achieved directly through the `pre_gen.dart` hook. However, if the prompting logic is moved to the pre generation hook the `brick.yaml` will no longer need to declare the variables.

**Requirements**

- [ ] Support conditional prompting of variables whilst still declaring them

**Additional context**

- An effort like this one is likely to require a proper proposal document.