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.

This replaces PR #6786, which was targeting Svelte 3.

HEADS UP: BIG RESTRUCTURING UNDERWAY

The Svelte repo is currently in the process of heavy restructuring for Svelte 4. After that, work on Svelte 5 will likely change a lot on the compiler aswell. For that reason, please don't open PRs that are large in scope, touch more than a couple of files etc. In other words, bug fixes are fine, but feature PRs will likely not be merged.

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

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[rmunn]

Note that @dummdidumm assigned #6786 to the "One Day" milestone on March 20. This one will probably also go in the same milestone.

[vercel]

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[Rich-Harris]

Closing Svelte 4 PRs as stale — thank you