Consider adding a warning when assigning values to arrays without incrementing the index

[mint-thompson]

Transferred from internal JIRA issue originally created 2023-11-03.

For example:

* entry[SomeSlice] = Reference(my-first-resource)
* entry[SomeSlice] = Reference(my-second-resource)

In this case, it is likely the author meant to write:

* entry[SomeSlice][+] = Reference(my-first-resource)
* entry[SomeSlice][+] = Reference(my-second-resource)

Consider adding this warning. We should think about how this would affect different FSH types. If we only want to warning when assigning a value to the same path of an array, then this would only affect Instances. That could be good since CodeSystems tend to get very long and this could potentially add a lot of time to process them.