Add Step and Precision for groupsockets

Hey, there's this neat thing in custom properties in Blender, "Step" and "Precision". The step lets you control the value (by dragging it) faster and slower and the precision is a visual representation of more and fewer digits. i.e 0.102, 0.10, 0.1

So for example you're creating some node group with some float value and you want to set it between 0 - 0.1, you would put like 4 digits precision and a low step and let the user control it in a more precise manner :)

Edit: typo