nonfree: Add multimc.

• nonfree/packages/minecraft.scm (multimc): New variable.

This MR adds multimc, built from source but with some nested dependencies.

The build procedure forces us to build these as embedded source, so unbundling would not give us major benefits.

You still need a Mojang account to receive credentials that will subsequently be used to download both the game and its assets.

If this would rather go in the nonguix repo because it is technically not an actual game, I am also open to that.