fix: reflect SvelteURLSearchParams changes to SvelteURL

[FoHoOV]

Currently any change to SvelteURL.searchParams is not reflected on SvelteURL directly. This PR separates url-search-params and url into different files (just like other classes) and adds more tests (also reduces the number of redundant updates).

fixes URL part of #11727 (meaning this where changes to search params is not reflected on url)

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[FoHoOV]

Ofc, why not, node 18.20s implementation is somehow screwing with me. Maybe its using ctx.search instead of ctx.searchPrams? Idk I'll look into it later :(

Edit: yep it does, searchParams are in sync, but url.search is not in sync with url.searchParams 🗡️ 🗡️ 🗡️ they use different things internally on node18.20