Update astro add to install stable peer dep

[bluwy]

Changes

With the added Svelte 5 (alpha) support for @astrojs/svelte, when users run astro add svelte, it will incorrectly install [email protected]:

  Astro will run the following command:
  If you skip this step, you can always run it yourself later

 ╭────────────────────────────────────────────────╮
 │ pnpm add @astrojs/svelte svelte@^5.0.0-next.1  │
 ╰────────────────────────────────────────────────╯

This PR fixes it by only installing the latest non-prerelease version:

  Astro will run the following command:
  If you skip this step, you can always run it yourself later

 ╭─────────────────────────────────────────╮
 │ pnpm add @astrojs/svelte svelte@^4.0.0  │
 ╰─────────────────────────────────────────╯

(Originally reported from the Svelte discord)

Testing

Tested running astro add svelte locally.

Docs

n/a. bug fix.

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Latest commit: 6ae53fd

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[bluwy]

Ugh I just realized #9387 also fixes this 😬