fix: install @bettter-auth/cli as a dev dependency

[sacrosanctic]

closes #937

in this pr:

• remove reliance on a package manager by installing the cli as a dev dependency
• update pnpm install instructions to use corepack (src / src)
• use pnpm dlx (src)
• lock create and add args to a specific version for reproducibility

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-cli-950-svelte.vercel.app/

[changeset-bot]

🦋 Changeset detected

Latest commit: 03fc947

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

This PR includes changesets to release 1 package

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sv	Patch

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[pkg-pr-new]

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npx https://pkg.pr.new/svelte-migrate@950

npx https://pkg.pr.new/sv@950

npx https://pkg.pr.new/@sveltejs/sv-utils@950

commit: 84bb18f

[paoloricciuti]

An even better way could be to add @better-auth/cli as a dev dependency and have the command be better-auth generate --config src/lib/server/auth.ts --output src/lib/server/db/auth.schema.ts --yes...just tested and it works (and I prefer that to always install)

[WarningImHack3r]

Suggested change

	fix: install `@bettter-auth/cli` as a dev dependency
	fix: install `@better-auth/cli` as a dev dependency

;)

[WarningImHack3r]

Even though as per your link in the PR description, pnpx is not always there by default, I'm pretty sure it exists and is both used and preferred by the vast majority of pnpm users, and probably won't go away anytime soon. I'd argue to leave it as it was, or at least invert the original commands ("pnpm dlx ... or pnpx ...")

[WarningImHack3r]

Maybe add a line or two to explain why specifically use corepack over the user's install, if it even is mandatory to use the corepack version?
Your sources explain why corepack is still a viable install method, but don't mention any specific advantage over any other installation method. Maybe removing this block altogether and let the user install it however they want is simpler to manage?