feat: `Item Number of () in ()` (index query) for fields of a struct list

[aitchFactor]

Struct lists can't currently be index queried even though the idea is pretty simple. If I have the following definitions:

struct MyStruct {
    name,
    date,
}

list MyStruct my_struct_list;

Then something like this would be useful, but isn't currently possible:

index = "02/02/2026" in my_struct_list.date;

Which would compile to this:

Thanks!