feat(nextjs): Tag backend events when running on vercel

[lobsterkatie]

Because nextjs apps behave somewhat differently on vercel than off, it's helpful to know when an event came from a vercel-deployed app vs. a non-vercel-deployed app. This adds a tag to events from vercel, based on an environment variable which vercel sets in the serverless environment in which backend lambdas run.

Note: If the user has overridden the default and turned off Automatically expose System Environment Variables in their project settings, we won't know they're on vercel. That said, it's hard to know why one ever would, and likely most people don't, so presumably this data will be close enough to accurate to remain useful. See https://vercel.com/docs/concepts/projects/environment-variables#system-environment-variables.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	22.47 KB (0%)
@sentry/browser - Webpack	23.35 KB (0%)
@sentry/react - Webpack	23.38 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	29.92 KB (0%)

[iker-barriocanal]

Instead of vercel: true, what do you think if we add a more generic tag like context: vercel? This will allow us in the future to add more values without adding more tags, which will also make queries simpler (and easier to not leave any data out of the results if we have more tag formats).