Add an ability to create member groups for functions, variables, and signals in the members' window in Visual Script

### Describe the project you are working on
I'm working on a 3D endless runner game.

### Describe the problem or limitation you are having in your project
The project's code is quite complex than I originally thought. As the project grows, my functions, variables, and signals increased in number. Many non directly related functions or variables are mixing together. Therefore I have a hard time finding and managing them in the members' window.

### Describe the feature / enhancement and how it helps to overcome the problem or limitation
By adding an ability to create member groups for functions, variables, and signals in the members' window, this issue will be fixed. Any related functions, variables, and signals can be grouped together, hence will be easy to find.

### Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams
Here's my mock-up of the members' window with this proposal where functions, variables, and signals can be grouped together:


### If this enhancement will not be used often, can it be worked around with a few lines of script?
This enhancement will be used all the time and it can't be worked around with a few lines of script as it's the core feature of the engine.

### Is there a reason why this should be core and not an add-on in the asset library?
This is the core functionality.