[6.x] Update setup-cp-vite to install Vite 8

[jasonvarga]

Fixes #14558.

A fresh install of Statamic uses Vite 8, but running statamic:setup-cp-vite was regressing the version back to Vite 7.

Changes

• Bumped the version installed by statamic:setup-cp-vite from ^7.0.4 to ^8.0.0 so it matches what the starter kit ships with.
• Fixed the existing-dependency detection. The check used Collection::contains('vite'), which inspects values, not keys. Since dependencies/devDependencies are keyed by package name with version strings as values (e.g. "vite": "^8.0.0"), the check always evaluated to false and the command always overwrote whatever version was already installed. Switched to Collection::has() so an existing vite (or @statamic/cms) dependency is left alone. This is what was causing the silent regression on fresh installs.
• Added a regression test covering the case where both packages are already present.

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