[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.

This PR is based on sveltejs#6750 since sveltejs#6750 also changes the function signature of the derived callback. If this PR was merged without sveltejs#6750, then it would make sveltejs#6750 more difficult to merge since the derived callback would then have to have the signature (stores, set, changedArray, update) which is much uglier than (stores, set, update, changedArray). So I want to wait for sveltejs#6750 to be merged (or rejected) first, then a decision can be made on this PR.

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Tests

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