refactor: switch to an incremental source generator (#720)

Use an [incremental source generator](https://github.com/dotnet/roslyn/blob/main/docs/features/incremental-generators.md) (see [here](https://andrewlock.net/creating-a-source-generator-part-1-creating-an-incremental-source-generator/)) for creating the test classes.

Replace inheritance with a marker attribute, so for the test classes you have to write
```csharp
[FileSystemTests]
public partial class Tests
{
    // your tests...
}
```
instead of
```csharp
public abstract partial class Tests<TFileSystem>
	: FileSystemTestBase<TFileSystem>
	where TFileSystem : IFileSystem
{
    // your tests...
}
```

Directory.Packages.props

@@ -23,7 +23,7 @@
   </ItemGroup>
   <ItemGroup>
     <PackageVersion Include="Microsoft.CodeAnalysis.CSharp" Version="4.12.0" />
-    <PackageVersion Include="Microsoft.CodeAnalysis.Analyzers" Version="3.3.4" />
+    <PackageVersion Include="Microsoft.CodeAnalysis.Analyzers" Version="3.11.0" />
     <PackageVersion Include="Microsoft.Extensions.DependencyInjection" Version="9.0.0" />
   </ItemGroup>
   <ItemGroup>