Sub States Deprecation

We're planning to remove the option to declare sub-states on the state using the children property. This feature was introduced years ago to address certain issues, but it's not technically beneficial and doesn't add any value.

Moreover, this feature has been restrictive because you always had to consider state erasure when calling ctx.setState in states that have sub-states. This is because ctx.setState removes the sub-state, which is mostly invisible from the parent state.

Therefore, it necessitated maintaining a relationship between a parent and a child state.

It's preferable to have separate states that maintain the relationship. There shouldn't be bidirectional dependencies between states, as it often leads to cyclic dependency errors.

For example, if you have an invoice state and an invoice lines state, instead of tying the invoice lines state to the invoice state through children: [InvoiceLinesState], you can keep InvoiceLinesState entirely separate. Each invoice line item may have an invoiceId property. Then, when you want to select a list of line items for an invoice, you can join the line items state selector with InvoiceState.getInvoiceId or something similar to retrieve the invoice ID, and then filter the invoice lines list by invoiceId.