ref: Hoist flush, close, and lastEventId into @sentry/core

[lforst]

The current duplicated implementation of flush, close and lastEventId makes those functions hard to work with in "isomorphic" SDKs like Next.js and Sveltekit.

A simple example: We could make our wrapping methods in Next.js isomorphic, however, these functions make use of flush so we would need to import either from @sentry/browser or @sentry/node which goes a bit against the whole isomorphism.

This PR hoists these functions up into @sentry/core so that we can access them isomorphically.

There is one thing going on that's worth wasting one or two brain cells on, namely that we change the return types of flush and close from PromiseLike to Promise in the browser SDK, but that should be fine since Promise obviously implements PromiseLike.

On that note, because I mentioned "isomorphic" a lot, I decided to google it to check if it was actually a thing and it even has its own wikipedia article so we gucci: https://en.wikipedia.org/wiki/Isomorphic_JavaScript

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	74.86 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.11 KB (0%)
@sentry/browser - Webpack (gzipped)	21.81 KB (0%)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	69.54 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	28.15 KB (+0.05% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	20.17 KB (+0.13% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	219.06 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	84.67 KB (+0.05% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	59.83 KB (+0.07% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.02 KB (+1.94% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	65.05 KB (-0.01% 🔽)
@sentry/react - Webpack (gzipped)	21.84 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	92.65 KB (+0.01% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	50.61 KB (-0.02% 🔽)