Allow managed types with .Ctx() for systems/observers, and update AppBuilder.Init() callback signature

[BeanCheeseBurrito]

This PR allows managed types to be passed as user context to systems and observers. Previously only pointers to unmanaged types were allowed.
Old:

int value = 10;

world.System()
    .Ctx(&value) // Pointer has to be passed.
    .Each(static (Iter it, int _) =>
    {
        ref int ctx = ref it.Ctx<int>();
        Console.WriteLine(ctx); // Prints 10
    });

New:

world.System()
    .Ctx("Context") // Context is passed by value
    .Each(static (Iter it, int _) =>
    {
        ref string ctx = ref it.Ctx<string>();
        Console.WriteLine(ctx); // Prints "String"
    });

A callback can be provided to run clean-up logic before the context object is freed.

world.System()
    .Ctx("Context", static (ref string ctx) =>
    {
        Console.WriteLine("Context is being freed");
    }) 
    .Each(...);

The callback signature for AppBuilder.Init() has been changed to accept a World instead of ecs_world_t*

Old:

using World world = World.Create();

world.App()
    .Init(Setup)
    .Run();

static void Setup(ecs_world_t* worldPtr)
{
    World world = World.Create(worldPtr);
    world.Import<Module1>();
    world.Import<Module2>();
    world.Import<Module3>();
}

New:

using World world = World.Create();

world.App()
    .Init(Setup)
    .Run();

static void Setup(World world)
{
    world.Import<Module1>();
    world.Import<Module2>();
    world.Import<Module3>();
}