docs: lowercase npm and pnpm

[Rich-Harris]

'npm' should never be capitalized; the same goes for pnpm. (I also moved Yarn to the bottom of the list, reflecting its priority)

[changeset-bot]

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Latest commit: 4b7282c

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-cli-490-svelte.vercel.app/

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commit: 4b7282c