Entity ids should start at one instead of zero

[evilfactory]

It's kinda annoying that the default value of an Entity or EntityReference is a valid entity. For example if I have:

World.Create();

EntityReference entity = default;
if (World.IsValid(entity))
{
    Console.WriteLine("valid");
}

This will pass as a valid entity.

My proposal is to have the ids start at one, so the default struct will always be an invalid entity.