Improve `ResourceIndex::update_one()` for More Detailed Events

[tareknaser]

Currently, ResourceIndex::update_one() returns a Result<IndexUpdate<Id>>, which basically just includes a couple of hashsets for added and removed resources. This is simple so that it matches the return type of update_all().

However, we should consider enhancing update_one to provide more detailed events. This could make the Track API and Watch API more powerful compared to the Reactive API. The goal is to support finer-grained events, similar to what notify-rs offers, including not just add/remove events but also rename and modify events.

For more context, see #36 (comment)

[kirillt]

Also, we can consider returning enum from update_one (instead of struct), since only one path can be updated. But this is only possible if we have dedicated enum variants for modifications and renamings.