[types] Fix signature of steps and mix functions

[MysteryBlokHed]

Fixes #322

This PR fixes the definitions of steps and mix on the Color class, which previously didn't have the correct return type.

[lloydk]

I don't think this change is correct. The steps function really does return PlainColorObject[] and the steps method on the Color class converts the PlainColorObject[] to Color[]

The Color class defines the steps methods here: https://github.com/LeaVerou/color.js/blob/2bd19b0a913da3f3310b9d8c1daa859ceb123c37/types/src/index.d.ts#L72
Those methods have the wrong return type (PlainColorObject[] instead of Color[]).

Not sure what the correct fix should be, maybe inline the method definitions instead of using ToColorPrototype so the return type can be changed to Color[]?

[lloydk]

The mix methods on the Color class have the same issue. They have a return type of PlainColorObject but the return type should be Color.