removed old code for svelte4 like dispatchers and also some unclear c…

[ildella]

In svelte 5 I Was getting this warning:

%c[svelte] legacy_recursive_reactive_block
%cDetected a migrated `$:` reactive block in `node_modules/svelte-marked/src/SvelteMarkdown.svelte` that both accesses and updates the same reactive value. This may cause recursive updates when converted to an `$effect`.
https://svelte.dev/e/legacy_recursive_reactive_block

this PR fixes it removing the now useless event dispatch code.
Also removed more unused code and replaced legacy/run with new svelte5 rune $effect