Reactive writable URL Search Params with Svelte 5 runes

Describe the problem

Storing state in the URL is a common practice and a recommended approach for state management in SvelteKit. Adding a way to easily change the search parameters of the current URL was previously discussed in issue #969, but this discussion predates the introduction of Svelte 5 runes.

With runes, keeping $state variables reliably synchronized with URL search parameters is challenging. page.url.searchParams can be used to read search params state, but not to write it, so we have to ressort on a sophisticated logic with $effect, snapshot, etc.

While community packages like sveltekit-search-params exist to address this pattern, the most popular one is not yet based on runes (see paoloricciuti/sveltekit-search-params#80 and relies on the deprecated $page store. I don't know any popular library that solve this issue the latest rune based APIs

I think SvelteKit should provided a built-in, rune-native, and documented way o manage this synchronization.

Describe the proposed solution

Integrate something like https://gist.github.com/PuruVJ/ed0abefbf512ff2aaa5dbbd377c50f5a by @PuruVJ into the framework.

Alternatives considered

No new api, but a new documentation page about how to read-write state in the URL.

Importance

nice to have

Additional Information

No response

Start by reviewing issue #969, the linked gist, and closed pull request #16031 to understand prior discussion and attempted direction. Done means maintainers have settled whether a rune-native built-in API or documentation-only solution is wanted and have specified its behavior and scope.