Add subgraphing in VisualScript to create modular nodes - GSoC'22

[VikramSGIT]

Hey There! I'm Vikram S. I would like participate as GSoC'22 contributor for Godot.
I've made a PR to showcase my ideas.

I'm taking Blender3D's VisualShading as my inspiration. Included features are:

1. Compact view of the function.
2. Can share the Node with community.

Other ideas in my mind:

1. To have multiple views Compact and a look-up view:
   • Compact: Normal function.
   • look-up: Have the editor rendered into a small frame.
2. Implement a holder, with which Nodes can be grouped. Also to implement commenting feature (as mentioned on Making visual script easier to organize or put in compartments . . #3943).

PhotoShoped!!

Thank you ❤️