C#: Make editor create NuGet fallback folder for Godot packages

Main benefits:
- Projects can be built offline. Previously you needed internet
  access the first time building to download the packages.
- Changes to packages like Godot.NET.Sdk can be easily tested
  before publishing. This was already possible but required
  too many manual steps.
- First time builds are a bit faster, as the Sdk package doesn't
  need to be downloaded. In practice, the package is very small
  so it makes little difference.

Bumped Godot.NET.Sdk to 4.0.0-dev3 in order to enable the
recent changes regarding '.mono/' -> '.godot/mono/'.

modules/mono/editor/GodotTools/GodotTools.ProjectEditor/ProjectGenerator.cs

@@ -3,14 +3,13 @@ using System.IO;
 using System.Text;
 using Microsoft.Build.Construction;
 using Microsoft.Build.Evaluation;
+using GodotTools.Shared;
 
 namespace GodotTools.ProjectEditor
 {
     public static class ProjectGenerator
     {
-        public const string GodotSdkVersionToUse = "4.0.0-dev2";
-
-        public static string GodotSdkAttrValue => $"Godot.NET.Sdk/{GodotSdkVersionToUse}";
+        public static string GodotSdkAttrValue => $"Godot.NET.Sdk/{GeneratedGodotNupkgsVersions.GodotNETSdk}";
 
         public static ProjectRootElement GenGameProject(string name)
         {