Investigate `filename` reporting

[bitsandfoxes]

The SDK is setting filename to just the file name (e.g., "SomeMonoBehaviour.cs") instead of the relative path from project root (e.g., "Assets/Scripts/SomeMonoBehaviour.cs").
This breaks code mapping functionality in Sentry.

On Mono, the frame.FileName resolves to the absolute path.
On IL2CPP the frame.FileName resolves to the absolute path.

[bruno-garcia]

In .NET we have a build time thing that sets the project path to an attribute (C#), which we can read at runtime to find what we should be cutting off of the path.

We could do the same with Unity since we run code during the Editor build.