feat(node): Add checkin envelope types

[AbhiPrasad]

ref getsentry/sentry#43661

I had some time on my hands, so added this in. Does not add any functionality around sending check-ins, only the envelope types needed to construct them.

https://develop.sentry.dev/sdk/check-ins/

I overengineered envelope types 😭

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	20.94 KB (+0.08% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	65.44 KB (+0.03% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.49 KB (+0.07% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	57.88 KB (+0.04% 🔺)
@sentry/browser - Webpack (gzipped + minified)	21.11 KB (+0.06% 🔺)
@sentry/browser - Webpack (minified)	68.87 KB (+0.03% 🔺)
@sentry/react - Webpack (gzipped + minified)	21.14 KB (+0.07% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	48.9 KB (+0.03% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	28.5 KB (+0.06% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.73 KB (+0.06% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	44.92 KB (0%)
@sentry/replay - Webpack (gzipped + minified)	38.87 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	63.82 KB (+0.03% 🔺)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	56.85 KB (+0.03% 🔺)