[csharp][booking][01_base] Put projects under Solution Folders

Author: solvingproblemswithtechnology

examples/csharp/csharp-booking-01_base/csharp-booking-01_base.sln

@@ -3,9 +3,13 @@ Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio Version 16
 VisualStudioVersion = 16.0.30626.31
 MinimumVisualStudioVersion = 15.0.26124.0
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Booking", "src\Booking\Booking.csproj", "{E268D5F9-53C4-4AFE-8726-A69B0405E97C}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Booking", "src\Booking\Booking.csproj", "{E268D5F9-53C4-4AFE-8726-A69B0405E97C}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Booking.Tests", "test\Booking.Tests\Booking.Tests.csproj", "{5401F189-E054-490C-A525-8ADC69FC6717}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Booking.Tests", "test\Booking.Tests\Booking.Tests.csproj", "{5401F189-E054-490C-A525-8ADC69FC6717}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{2081C16F-3A69-43B6-A2EC-B2DC50FB5AEF}"
+EndProject
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "test", "test", "{A10B43E7-C43C-4A18-9DD6-5010B8530106}"
 EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
@@ -45,6 +49,10 @@ Global
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
 	EndGlobalSection
+	GlobalSection(NestedProjects) = preSolution
+		{E268D5F9-53C4-4AFE-8726-A69B0405E97C} = {2081C16F-3A69-43B6-A2EC-B2DC50FB5AEF}
+		{5401F189-E054-490C-A525-8ADC69FC6717} = {A10B43E7-C43C-4A18-9DD6-5010B8530106}
+	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {A9B83075-4808-4A9E-8C77-26CC1263045F}
 	EndGlobalSection