Allow setting a load order for implicit layers

[DadSchoorse]

For explicit layers it's easy to manage the order with the VK_INSTANCE_LAYERS env var but currently there is no way to specify which implicit layer should be loaded first. Such a way would be useful for making sure that post processing layers like reshade or vkBasalt get applied before the steam overlay / other overlay layers. As for how to achieve this, my proposal would be a signed integer value in the manifest file to specify the load priority for each layer.