Shared state mechanism

[Avokadoen]

State that is not connected to the entity should be able to be expressed and shared across systems

Suggested API:

// writing to shared state should be allowed and thread safe 
// scheduler will avoid dispatching these in parallel
fn systemWithSharedStateMutate(a: A, window: Shared(*WindowHandle)) void {
    window_api.setCursor(.default);
    _ = a;
}

// read only shared state allow scheduler to do these in parallel!
fn systemWithSharedStateConst(a: A, color: Shared(CommonColors)) void {
    a.color = color.green;
}

var world = WorldBuilder().WithSystems(
    systemWithSharedStateMutate,
    systemWithSharedStateConst,
).WithSharedState(
    WindowHandle,
    CommonColors,
).init(allocator);

world.registerShared(WindowHandle, 123);

try world.dispatch(); // error.UndefinedSharedState