Move the dll to BepInEx\core so plugins don't have to bundle their own copies

[ManlyMarco]

API assemblies that aren't plugins can be copied to BepInEx\core to make them accessible from everywhere. This would make it no longer necessary to bundle this lib with your other plugins, just list it as a dependency. Alternatively you can make an empty plugin class so that BepInEx will always load this assembly and other plugins will be able to use BepInDependency to ensure this exists and is loaded first.