Add a sign with mushroom planks design.

It is very difficult, because the vanilla sign has a hard coded type definition. These type defines which item will be dropped and which texture the sign has.

So maybe a forge extension of the sign is a possibility or copy all sign related classes and add own type.

1.16.5-36.0.60 Support for Custom Wood Type (a bug with sign textures was fixed in: 1.16.5-36.1.1)

will be released in 1.6.0