[WIP] feat(nextjs): Add example app for tests

[lobsterkatie]

Not yet working.

• Add example app based on https://github.com/vercel/next.js/tree/master/examples/with-typescript
• Add next.config.js, sentry.server.config.js, sentry.client.config.js to test app (that last one is empty right now).
• Create script for setting up environment for testing - linking SDK, building test app. Working.
• First pass at starting server programmatically and running tests against it. Not working. I suspect I'm having async problems.

NB: The last commit is just a copy of #3555 and can go away once that's merged.

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	20.75 KB (0%)
@sentry/browser - Webpack	21.97 KB (0%)
@sentry/react - Webpack	22 KB (0%)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	28.13 KB (0%)

[AbhiPrasad]

Closing because #3632 has been merged in