Add source generator test file

Author: dibarbet

test/lsptoolshost/integrationTests/testAssets/slnWithCsproj/src/app/SourceGenerator.cs

@@ -0,0 +1,17 @@
+using System;
+using System.Text.Json.Serialization;
+
+namespace SourceGeneration;
+
+public class WeatherForecast
+{
+    public DateTime Date { get; set; }
+    public int TemperatureCelsius { get; set; }
+    public string Summary { get; set; }
+}
+
+[JsonSourceGenerationOptions(WriteIndented = true)]
+[JsonSerializable(typeof(WeatherForecast))]
+internal partial class SourceGenerationContext : JsonSerializerContext
+{
+}