fix: make deriveds on the server lazy again

[dummdidumm]

Fixes a regression introduced in #15820: deriveds need to be lazily called on the server, too, since they can close over variables only later defined

Fixes #15960

After having opened this I realized that the new behavior fixed #15672, so I guess we need to adjust this PR to keep that fixed.

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[changeset-bot]

🦋 Changeset detected

Latest commit: dadff16

The changes in this PR will be included in the next version bump.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15964-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15964

[paoloricciuti]

I think we need to at least change

this.foo() = 3;

be

this.foo(3);

However I wonder what are the implications of doing an assignment in the constructor (that now needs to create a new derived)