feat: pass changed boolean array to derived callbacks

[rmunn]

This allows the callbacks to easily run complex update logic based on which store changed most recently.

Fixes #6777.

NOTE: I have based this PR on top of #6750 since #6750 also changes the function signature of the derived callback, and if a changed array was added to the callback function signature and then #6750 was merged later, the signature would become (stores, set, changedArray, update) which is much uglier than (stores, set, update, changedArray). That is also why this is a draft PR; once #6750 is either merged or rejected I'll convert this PR from a draft to a real PR (and also rebase it so there are no merge conflicts).

Basing this PR on top of #6750 brings the unfortunate effect that it can be hard to see this PR's changes in isolation, so I've created rmunn#1 to see this PR's changes compared to the changes in #6750. Hopefully that makes things clear enough.

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.

Tests

• Run the tests with npm test and lint the project with npm run lint

[rmunn]

This PR is superseded by #8656, which merges against the version-4 branch.