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/GodotTools.ProjectEditor.csproj

@@ -10,6 +10,7 @@
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\GodotTools.Core\GodotTools.Core.csproj" />
+    <ProjectReference Include="..\GodotTools.Shared\GodotTools.Shared.csproj" />
   </ItemGroup>
   <!--
   The Microsoft.Build.Runtime package is too problematic so we create a MSBuild.exe stub. The workaround described