chore: add custom_renderer flag

[paoloricciuti]

This adds a flag to prevent runtime overhead when custom renderer is not enabled.

Should be merged before #18405

[changeset-bot]

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Latest commit: 0215a18

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-18461-svelte.vercel.app/

[github-actions]

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pnpm add https://pkg.pr.new/svelte@18461

[pkg-svelte-dev]

Install the latest version of svelte from 0215a18:

pnpm add https://pkg.svelte.dev/svelte/c/0215a18431ed53fda35fad930605507fb0632ab2

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/svelte/pr/18461