Provide utility buildfile in the package which contains useful targets to manipulate the Godot Engine

[Klaim]

In my project MEGASTRUCTURES I setup useful non-project-specific targets that can download and launch the Godot Engine executable (as it's self-contained) with the specified project. This is very useful in general to keep in sync the godot-cpp version a project uses and the Godot Engine executable version, but also to help building ad-hoc recipes that just build everything and then open the editor or launch the game or export it ("export" means making a publishable/installable release build, in Godot's jargon).