I am new to Godot. I'm creating a drawing board on Godot and would like to save and load the drawings as a PNG file. And once the drawing has been saved, I'd like to be able to open that saved drawing and be able to continue drawing on that saved board. Anybody know how I can go about doing that?

The Image class has a save_png() method you can use for this purpose. You can either manipulate the Image's contents directly or capture a viewport's data to the image (example).

How do I find 'EditorPlugin' in the Nodes? I'm getting an error saying: " Script inherits from native type EditorPlugin, so it cant be instanced in object of type: Node "