chore: stabilize experimental api

[bluwy]

Description

Closes #7653

Remove @experimental tag for stable APIs

Additional context

Added github codesearch results for checking API usage

Stabilizes:

• preview() (Usage) - Looks there's one usage only, though may still be good to export. I also updated the types for this function to match createServer()
• preTransformRequest (Usage) - Looks like Nuxt and vite-node are using them the most. The need was raised from tests run 3x slower than Jest vitest-dev/vitest#229 (comment) and feat: option to disable pre-transform #6309
• server.fs.deny (Usage) - Introduced for security concerns feat: server.fs.deny support #5378, and marked experimental because it may be a breaking change I believe. We can change this in Vite 3.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[bluwy]

Just noticed one more API we could look into, which is the ssr config option, which is marked as @alpha. I suppose it's safe to stabilize this as well since we last discussed?