fix: clean up scheduling system

[dummdidumm]

Consolidates our scheduling system that had diverged into two - queue_micro_task and Batch.ensure-queues and resolves some edge case race conditions related to flushSync along the way.

Extracted (and robustified) from #16709

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Tests and linting

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[changeset-bot]

🦋 Changeset detected

Latest commit: 67fd61c

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

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16741-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16741 

[Rich-Harris]

appreciate the detailed comment in task.js — took me a few reads to get my head round it but it makes sense

don't fully understand the transition test changes but i trust you on this