Show usages of serialised ScriptableObject assets

Rider will show usages of `ScriptableObject` based classes by showing the assets that the class is serialised into. It is more useful to show where that serialised asset is used.

The issue does not name files, tests, or entry points. Start by locating the Unity/Rider usage analysis for serialized ScriptableObject assets, then determine how usages of those serialized assets should appear. Done means the usage view shows where the serialized asset is used rather than only where its class is serialized.

Written by the indexing model from the issue text.