Introduce GDscript main plugin script

[dogboydog]

https://github.com/paulloz/godot-ink/blob/main/addons/GodotInk/plugin.gd

This would help make it clearer for users what to do at least if the plugin can't be initialized due to C# not being built.

[dogboydog]

I noticed that in at least Godot 4.3, if I compile c# after checking out a project, the plugin seems to re-enable itself fully. So the aspect of the half-enabled state as referenced in the screenshot doesn't seem to be that much of an issue to me. I'm not sure whether to make the GDScript main plugin script or not at this point.