Add a VisualScript state node

**Describe the project you are working on:**

Godot Engine (proposal from reduz)

**Describe the problem or limitation you are having in your project:**

Users can not make visual state flows

**Describe the feature / enhancement and how it helps to overcome the problem or limitation:**

The goal for this node is so users can make visual state flows for games, like screen interface chart, or a state chart during a level.

**Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:**


**If this enhancement will not be used often, can it be worked around with a few lines of script?:**

Not a few lines of code.

**Is there a reason why this should be core and not an add-on in the asset library?:**

Visual Script is coded in c++. Might be possible to be a gdscript node, except this function is special due to the fact it calls _process and _physics_process.