Quick-fix to apply a FormerlySerializedAs to serialized values in assets, prefabs and scenes

[oxysoft]

Opening the quick-fix popup with the caret on a FormerlySerializedAs attribute could offer the user to instantly apply the new name to all uses of the old identifier.

This would be a very awesome feature, as right now it's easy for a codebase to become polluted with this attribute over time. Without manual investigation, it's never clear whether removing the attribute would be safe or if it would lose data.

The IDE could either do this manually by searching for the script's GUID and then parsing the YAML for the results, or use its existing index if "Parse text based assets" is enabled.