Reject creating property with name that case-insensitively matches an existing property

[lognaturel]

It's currently possible to create multiple properties that only differ by case: https://staging.getodk.cloud/#/projects/103/entity-lists/cacti

It's unlikely that users actually want such duplicate names. I think it's pretty easy to accidentally end up in this state if multiple form definitions update the same properties by introducing a case typo in one of them.

I believe we already reject Entity List names that are case-insensitive duplicates. I think we should do the same for property names.