[UI-270] Adds new option shouldValidateBranch to createValidatedForm for helping to create branching forms.

[George-Payne]

When a parent validated form is validated, it will only call validation on a nested validated form if branchIsActive returns true (or is undefined).
branchIsActive is passed the data of the top level form which had validation called on it, it's own data, and the reason for validating.

Example usage

interface DinnerForm {
    mealType: string;
    pizzaToppings: {
        pepperoni: boolean;
        pineapple: boolean;
    };
}

const form = createValidatedForm<DinnerForm, DinnerForm>({
    mealType: '',
    pizzaToppings: createValidatedForm<DinnerForm['pizzaToppings'], DinnerForm>(
        {
            pepperoni: true,
            pineapple: true,
        },
        {
            branchIsActive: (root) => root.mealType === 'pizza',
        },
    ),
});