Add new generator for forks

[ChillerDragon]

There are some newer clear structures for forks that can cleanly separate them. They have their own chat, rcon and variable headers and files that are included in some central include only file. And the entire code should be in src/forkname same as it is src/insta with a own CMakeLists.txt.
Keeping thing really clear. If the cli helper would encourage that shape it can help users follow it. Also I personally would have needed for one side project.

So basically creating files like these:

src/forkname/server/chat_commands.h
src/forkname/server/chat_commands.cpp

And patching all the files in src/insta/includes/*