feat: abstract task logic to core

[paoloricciuti]

A bit of spare time today...i moved all the logic for task into createTask inside a core.ts file and used an adapter pattern to communicate between the logic and the layer that build the integration with svelte stores.

This will make it much easier to switch to runes or go agnostic if we decide too.

I've used camelCase for this since we might endup exposing this function.

One thing that i don't properly like about this approach is that each adapter will need to add the handlers in it's module, i've tried abstracting that away but i got owned by TS.

I've also renamed the derived states to camelCase so if we merge this before the docs we can more easily work on adding the other derived states (which btw i think will need a bit of rework since the general loading state should be derived from all the loading of the instance).