Closes#16211. Changes are mostly tests.

Currently mutating signals local to derived is allowed for primitives, arrays and objects, but not for sets and maps. Since the goal of derived is to be pure, mutating signals created inside it does not violate purity anyhow. This PR makes behavior consistent for sets and maps. Also covering all the other cases with tests to ensure future consistency.

constworks=$derived.by(()=>{letret=$state([])ret.push(1)returnret})constthrows=$derived.by(()=>{letret=newSvelteSet()ret.add(1)returnret})

Update: also applied the change to SvelteDate, SvelteURL and SvelteURLSearchParams

Tests are passing, butpnpm lint is crashing withError: Cannot find module ' /Code/svelte/node_modules/.pnpm/svelte-eslint-parser@0.43.0_svelte@packages+svelte/node_modules/svelte/compiler/index.js'. Not sure what to do about it.

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• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC:https://github.com/sveltejs/rfcs
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• Ideally, include a test that fails without this PR but passes with it.
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Tests and linting

• Run the tests withpnpm test and lint the project withpnpm lint