feat(core): Add top level getClient() method

[mydea]

This can be used instead of getCurrentHub().getClient().

This prepares us also for a post-hub time, but makes sense generally IMHO, also simplifies user-facing interaction with the client (e.g. to lazy-add integrations etc).

This is a 1-1 replacement, so should be pretty straightforward.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	66.02 KB (+0.12% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	56.11 KB (+0.16% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.11 KB (+0.29% 🔺)
@sentry/browser - Webpack (gzipped)	21.41 KB (+0.39% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	62.41 KB (+0.1% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	29.24 KB (+0.35% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	21.37 KB (+0.43% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	196.84 KB (+0.12% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	88.73 KB (+0.33% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	63.69 KB (+0.46% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	31.98 KB (+0.35% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	66.32 KB (+0.13% 🔺)
@sentry/react - Webpack (gzipped)	21.45 KB (+0.39% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	83.09 KB (+0.12% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	48.26 KB (+0.21% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	16.19 KB (+0.55% 🔺)