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/Build/BuildManager.cs

@@ -7,7 +7,6 @@ using JetBrains.Annotations;
 using static GodotTools.Internals.Globals;
 using File = GodotTools.Utils.File;
 using OS = GodotTools.Utils.OS;
-using Path = System.IO.Path;
 
 namespace GodotTools.Build
 {
@@ -209,6 +208,16 @@ namespace GodotTools.Build
             if (!File.Exists(GodotSharpDirs.ProjectSlnPath))
                 return true; // No solution to build
 
+            try
+            {
+                // Make sure our packages are added to the fallback folder
+                NuGetUtils.AddBundledPackagesToFallbackFolder(NuGetUtils.GodotFallbackFolderPath);
+            }
+            catch (Exception e)
+            {
+                Godot.GD.PushError("Failed to setup Godot NuGet Offline Packages: " + e.Message);
+            }
+
             GenerateEditorScriptMetadata();
 
             if (GodotSharpEditor.Instance.SkipBuildBeforePlaying)