Refactor lazyload to avoid unnecessary object creation

[MacFJA]

The current implementation create multiple IntersectionObserverCallback and IntersectionObserver, which are not needed.

[swyxio]

i might be wrong, but i think action function bodies are only run once? @kevmodrome. only the update() gets run multiple times.

[MacFJA]

I made a small example: https://svelte.dev/repl/2903aeece4524f0a9a66db758503a887?version=3.29.4 (see the console)

The main body is call on every time the action is used.
The update function only if the value change after the initialisation

[swyxio]

awesome, thank you very much!