Proposal: Exclude `CompilerGeneratedFilesOutputPath` from Compilation by Default

#### **Problem**
When `CompilerGeneratedFilesOutputPath` is set in csproj, the .NET SDK emits source generator output to disk. However, these files are **not excluded from compilation by default**, leading to errors when running `dotnet build` a second time.

Currently, developers must manually add:
```xml

```
Without this, builds fail due to duplicate type definitions: https://andrewlock.net/creating-a-source-generator-part-6-saving-source-generator-output-in-source-control/#excluding-emitted-files-from-the-compilation

#### **Proposal**
The .NET SDK should **automatically exclude** the `CompilerGeneratedFilesOutputPath` from compilation when it is set, preventing these errors.

Would love to hear thoughts on whether this can be supported natively!