[display] use isIterable and isKeyValueIterable in toplevel sorting

In order to support `for`-loop generation for arbitrary types with `iterator()` / `keyValueIterator()` methods instead of only a few hardcoded types from the std lib, the language server needs these flags in `FieldCompletionSubject`.

Haxe can also use this internally for expected type sorting on `for (foo in ...)` and `for (key => value in ...)`.