Add the option to create a symlink in the "mods-unpacked" directory to an existing directory

[KANAjetzt]

There is a nice utility to implement this:

https://github.com/godot-extended-libraries/godot-next/blob/master/addons/godot-next/global/file_system_link.gd

[KANAjetzt]

Done for 3.x

• https://github.com/GodotModding/godot-mod-tool/pull/106