feat(express): Warn if express is not instrumented

[mydea]

This prints a warning if we detect that the express app passed to setupExpressErrorHandler is not instrumented.
This happens if you import express before calling Sentry.init:

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser	21.65 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing)	32.67 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing, Replay)	68.01 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	61.41 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing, Replay with Canvas)	72.05 KB (-0.01% 🔽)
@sentry/browser (incl. Tracing, Replay, Feedback)	84.15 KB (-0.01% 🔽)
@sentry/browser (incl. Feedback)	37.66 KB (-0.01% 🔽)
@sentry/browser (incl. sendFeedback)	26.21 KB (-0.01% 🔽)
@sentry/browser (incl. FeedbackAsync)	30.84 KB (-0.01% 🔽)
@sentry/react	24.33 KB (-0.01% 🔽)
@sentry/react (incl. Tracing)	35.63 KB (-0.01% 🔽)
@sentry/vue	25.48 KB (-0.01% 🔽)
@sentry/vue (incl. Tracing)	34.46 KB (-0.01% 🔽)
@sentry/svelte	21.78 KB (-0.01% 🔽)
CDN Bundle	24.13 KB (-0.01% 🔽)
CDN Bundle (incl. Tracing)	34.04 KB (-0.01% 🔽)
CDN Bundle (incl. Tracing, Replay)	67.71 KB (-0.01% 🔽)
CDN Bundle (incl. Tracing, Replay, Feedback)	72.92 KB (-0.01% 🔽)
CDN Bundle - uncompressed	70.99 KB (-0.01% 🔽)
CDN Bundle (incl. Tracing) - uncompressed	100.98 KB (-0.01% 🔽)
CDN Bundle (incl. Tracing, Replay) - uncompressed	210.58 KB (-0.01% 🔽)
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	223.87 KB (-0.01% 🔽)
@sentry/nextjs (client)	34.86 KB (-0.01% 🔽)
@sentry/sveltekit (client)	33.23 KB (-0.01% 🔽)
@sentry/node	146.27 KB (-0.02% 🔽)