Strongly type FormField and subclasses

[emteknetnz]

During the work to add FieldValidator's to FormField's, it was noted that there's a lack of strong typing on FormField classes

Adding strong typing to these classes will indirectly support validation work, as well any future work to have a cleaner API surface for standardised UX components

Acceptance criteria

• Add strong typing to the FormField class and subclasses
• Where it makes sense, tidy up the logic of methods such as setValue(), setSubmittedValue(), Value(), etc