feat: move projectId to env file

[Jesperpaulsen]

This PR improves the dev experience by moving the projectId to an .env file

Not a big problem, but makes the developing experience better by moving the projectId to an .env file, so you don't have to change the sanity.json every time.

Requires the SANITY_STUDIO_API_PROJECT_ID to be set on vercel for the example project.

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🔍 Inspect: https://vercel.com/ricokahler/sanity-super-pane/2pfaJzYFaxMc4sdXBv6hiy1vKLQH
✅ Preview: https://sanity-super-pane-git-fork-fnatic-feature-imp-f938f5-ricokahler.vercel.app

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:tada: This PR is included in version 1.0.0-alpha.8 :tada:

The release is available on:

• npm package (@alpha dist-tag)
• GitHub release

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