Fixes: #16114

When a $state or a $derived is directly set by the user, we properly add it to the old_values Map.

However when a derived is updated as a result of the check_dirtiness check during flush_queued_effects, we do not save the old value, hence the value read in a teardown function of a $effect is not the old value but the new one, this is incosistent with the "old value behavior" that was recently introduced.

In this PR i save the old value when the $derived is updated, so that it will be used by teardown functions of $effects.

However while working on this i've noticed that there's an edge case when working with child components.

While passing a prop in certain conditions (either a legacy bindable or a prop accessed via $.prop because it has a setter somewhere in the code) the original $state is wrapped in a derived. When the child component is destroyed the teardown functions might read $.prop values that did not got update because thay were not read (perhaps in the template). This means that is possible that during the execution of the teardown the values of the derived are either null because they were never initialized because nothing ever read from it, or they are outdated. I've opened a separate issue for this: #16262.

This bug is present both before and after the changes of this PR.

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alternative to #16263, made possible by #16270. Closes #16114, closes #16262

Before submitting the PR, please make sure you do the following

• 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
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint