Replace "instanceof" check with something better

[nahkd123]

    Apparently esbuild got confused with ``state-machine`` from template-generated project and ``state-machine`` from ``file:../uikit`` (which is also a dependency of template-generated project) and decided to add ``Slot`` and ``Slot2``, making ``instanceof`` returns false. Might need to fix this in ``state-machine`` repository.

Originally posted by @nahkd123 in MixeryOSS/uikit#2 (comment)

[nahkd123]

I actually quite like the three.js style: putting isSomething = true to the class (Eg: Vector3 would be isVector3 = true if I remembered correctly).

Maybe something like _statemachine_isDynamicSlot = true for all dynamic slots (those with ability to send update events). Namespace is there to ensure that other projects can use isDynamicSlot without state-machine thinking it is an actual dynamic slot.