feat(core): Add client.init() to replace client.setupIntegrations()

[mydea]

This adds a new client.init() method to be called instead of client.setupIntegrations().

Note that this method simply initializes the integrations always, and depends on the check that we don't add integrations multiple times.

This also has a bit of a different semantic, dropping the force argument in favor of just calling addIntegration() again - this depends on #10116 to really work. cc @HazAT for Spotlight.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	76.86 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	68.23 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	61.85 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.26 KB (+0.08% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	30.75 KB (+0.09% 🔺)
@sentry/browser - Webpack (gzipped)	22.11 KB (+0.09% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	74.55 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	66.18 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	32.01 KB (+0.05% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	23.75 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	208.66 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	96.72 KB (+0.05% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	70.97 KB (+0.06% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	34.98 KB (+0.05% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	68.6 KB (+0.03% 🔺)
@sentry/react - Webpack (gzipped)	22.14 KB (+0.08% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	85.34 KB (+0.03% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	49.45 KB (+0.04% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	16.74 KB (0%)