Only inject Vercel analytics in production

[delucis]

Changes

• Only injects the script for Vercel analytics on build
• Vercel’s analytics scripts rely on some extra scripts Vercel injects magically to projects built on their platform but these aren’t available locally so 404 (reported in Vercel analytics file missing #6230 along with other stuff)
• Folks likely don’t want to track analytics locally anyway

Philosophical question: is this difference between dev and build OK? Generally differences between the two can trip people up — what if this script interferes with some other part of the project somehow and you only notice in production?

Testing

Existing tests should pass. Do we want a test that asserts the script is not included in dev?

Docs

No docs needed I don’t think.

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[bluwy]

I think it's fine for the dev and build to work differently, especially that analytics should be "addons" that a site can work with or without at a time.

It also looks like Vercel analytics doesn't have an API for tracking custom events, so the changes is safer too.