[FEAT] (Symbols) Ability to register new kinds of Symbols (i.e. new shape types for pv.Dot). pv.Scene.registerSymbol.
[FEAT] (pv.Dot aspectRatio) Dots now support the creation of rectangles and ellipsis, through the use of the new "aspectRatio" mark property.
[FEAT] (Symbols) Added new methods to allow inspecting registered symbols.