organize where dice skins live in the repo

[brimonk]

Is your feature request related to a problem? Please describe. I would like to contribute new dice skins. Currently, it is not clear where I should put the new skins, and how I should rig them up into the repo.

Describe the solution you'd like Ideally, there should be a directory under client/ specifically for skins, like client/skins. 2d dice will have a "2d" prefix, 3d dice will have a "3d" prefix. The prefixes will be followed by the name of the skin, and they should all be in the png format.

Describe alternatives you've considered I'm sure as long as it's organized, it's fine. If there are slight modifications to this, w/e.