Allow repopulating multi-selects from Laravel relationships

This commit adds support for repopulating multi-selects from a model's relationships.

For example, if your `$model` has a `roles` relationship, and you add a `Former::multiselect('roles')` field, Former will now repopulate the multiselect from the values set on that relationship.

Seeking code review from others about this approach before merging. I initially tried to reuse the logic in `Helpers::queryToArray()` but found it unsuitable for the task, so I rolled my own.