fix(playwright): move playwright install from prepare to test:e2e

[jycouet]

Closes #1062

Description

Move playwright install out of the prepare script and into test:e2e.

• prepare runs on every npm install, including in prod / CI environments that probably don't run e2e tests. No reason to download browsers there.
• It also fails noisily on unsupported systems (Arch, NixOS, ...) every time you add a dep.
• Moving it to test:e2e ties the install to the only place it's actually needed, without annoying the test launch (playwright skips when browsers are already cached).

Checklist

• Update snapshots (if applicable)
• Add a changeset (if applicable)
• Allow maintainers to edit this PR
• I care about what I'm doing, no matter the tool I use (Notepad, Sublime, VSCode, AI...)

[changeset-bot]

🦋 Changeset detected

Latest commit: c7986f2

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

This PR includes changesets to release 1 package

Name	Type
sv	Patch

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[pkg-pr-new]

Open in StackBlitz

npx https://pkg.pr.new/svelte-migrate@1072

npx https://pkg.pr.new/sv@1072

npx https://pkg.pr.new/@sveltejs/sv-utils@1072

commit: c7986f2

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-cli-1072-svelte.vercel.app/

[paoloricciuti]

What happens if the browsers are already installed, though? How fast it skips?

[jycouet]

It takes less than a second