Add instance_index option to parameters in visual shaders

[Chaosus]

• Closes Allow instance_index hint to be set inside a VisualShaderNodeParameter godot-proposals#9828
• Closes Boolean Instance Shader Parameters Cross-Conflicting with Multiple Materials on Same Mesh with Visual Shaders #112528
• Closes: Unique instance shader parameters from different shaders assigned to the same MeshInstance3D affect each other's values #107074

Adds "Instance + Index" option to parameter nodes.

[clayjohn]

Looks great!

[Repiteo]

Thanks!