Warn when overwriting an array entry when no index is provided

If authors do not specify an index on array paths, SUSHI will add the values at the first index. However, if an author adds a value at a path with an index (using either numeric indices or soft indices) and then uses the same path without the indices, SUSHI will overwrite the previously defined values with the values at the non-indexed path that came later.

Simplified FSH Online example: https://fshschool.org/FSHOnline/#/share/38nnIU9

Zulip thread: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/Validating.20extensions

Start by reproducing the behavior in the linked simplified FSH Online example, then trace SUSHI's array-path assignment handling for indexed and non-indexed paths. Review the linked Zulip discussion for context. Done means a later path without an index warns instead of silently overwriting values previously assigned with numeric or soft indices.

Written by the indexing model from the issue text.