use Spring class

[Thiagolino8]

The demo, by trying to use 2 different reactivity systems (runes and stores) needed an effect for synchronization and to ignore a warning about using state in a non-reactive way
Now that the Spring class exists, which uses runes and is much more powerful than the store, the demo can be simplified

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[manuel3108]

Hey, thanks for your work. We do already have a similar PR open, see #361 that we are likely to merge latest next week.

Thanks for your effort tho!

Closing as duplicate of #361