ref: Hoist createCheckinEnvelope to core package

[lforst]

Hoists the crons API to core so we can use it in non Node SDKs. We're hoisting it without increasing the browser bundle size by making the client method optional in the interface.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	21.01 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	65.62 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	19.55 KB (0%)
@sentry/browser - ES6 CDN Bundle (minified)	58.08 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	21.16 KB (0%)
@sentry/browser - Webpack (minified)	69.04 KB (+0.01% 🔺)
@sentry/react - Webpack (gzipped + minified)	21.18 KB (+0.01% 🔺)
@sentry/nextjs Client - Webpack (gzipped + minified)	49.1 KB (+0.03% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	28.64 KB (0%)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	26.86 KB (0%)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	47.35 KB (-0.01% 🔽)
@sentry/replay - Webpack (gzipped + minified)	41.13 KB (0%)
@sentry/browser + @sentry/tracing + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	66.3 KB (0%)
@sentry/browser + @sentry/replay - ES6 CDN Bundle (gzipped + minified)	59.18 KB (0%)