Not install GraphMakie when installing the library

[Tortar]

GraphMakie is an heavy dependency which means that even if it doesn't increase the load time of Agents since it is only used in the AgentsVisualizations extension, it does increase the time to install and precompile the library the first time. So it would be better to remove it from the dependencies in the Project.toml. But it's not clear how to do it without removing the possibility to use the package in the extensions.

Solutions:

• Make AgentsVisualizations its own package inside the main repo of Agents. This should work, but it's breaking.
• Adding a new extension loaded by GraphMakie, which extend the plotting of AgentsVisualizations. This should work also. And it is preferable since not breaking.

cc. @Datseris