feat: Add FiniteStateMachine

[idan]

Update: per @Hugos68 feedback, I converted to a class and renamed to FiniteStateMachine. Otherwise, no changes to the initial PR.

Adds a new useFSM rune that allows you to create minimalistic finite state machines.

Ported over from https://github.com/githubnext/tiny-svelte-fsm (which was already MIT, and I'm the author there). I made a couple of minor fixes along the way as well, mostly to enable people to call .debounce in actions and lifecycle handlers.

This PR includes:

✅ Docs with an interactive demo and examples ✅ Tests

Per our conversation in the Svelte Discord:

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[idan]

A lil' demo video of the demo at the top of the docs:

https://github.com/user-attachments/assets/fae59bbf-9252-4df5-b9f8-7e8e726b4960

[Hugos68]

Could you capitalize the types in the example code? It's a typescript convention.

[idan]

@TGlide thanks, what a lovely review.

I fixed all the things except for the generic vs type assertion. I don't have a better idea, but from a DX perspective, I definitely want the current state to be of type StateT not StateT | undefined or anything like that.