Unable to use Source Generator in VSTO with WPF

### Description

I want to use [ReactiveMarbles.ObservableEvents.SourceGenerator](https://www.nuget.org/packages/ReactiveMarbles.ObservableEvents.SourceGenerator/) in a VSTO project. The UI in the project is implemented using WPF. As long as a WPF User Control is added to the project, the project fails to build with an error: The type or namespace name 'ReactiveMarbles' could not be found (are you missing a using directive or an assembly reference?)

### Reproduction Steps

1. create an new Visio project with visual stuido
2. add nuget package ReactiveMarbles.ObservableEvents.SourceGenerator
3. add reference to presentationcore and presentationframework
4. create a new window with sample code from https://www.reactiveui.net/docs/handbook/events.html
5. add a new WPF User Control

### Expected behavior

successful build

### Actual behavior

The type or namespace name 'ReactiveMarbles' could not be found (are you missing a using directive or an assembly reference?)

### Regression?

_No response_

### Known Workarounds

_No response_

### Impact

_No response_

### Configuration

.NET version: .NET framework 4.8

### Other information

_No response_