feat(sveltekit): Export unstable_sentryVitePluginOptions for full Vite plugin customization

[Lms24]

As discussed with @lforst last week, we decided to provide an escape hatch to the reduced (but therefore version-agnostic) Vite plugin option API in the SvelteKit version of the plugin (sentrySvelteKit()). By specifying options in unstable_vitePluginOptions, users can still set and override all options the Vite plugin can take. We explicitly warn that keys in unstable_vitePluginOptions do not follow semver and might be removed or changed at any time. This allows
users to do whatever they need to and us to still bump Vite plugin major versions whenever we need to.

This PR now also adds an MIGRATION entry, which I previously forgot in #10813

builds on #10813
ref #9835

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.37 KB (+0.28% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	68.58 KB (+0.29% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	72.5 KB (+0.26% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.14 KB (+0.34% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.84 KB (+0.59% 🔺)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	32.84 KB (+0.59% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.34 KB (+1.09% 🔺)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.35 KB (+1.09% 🔺)
@sentry/browser - Webpack (gzipped)	22.56 KB (+1.4% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	75.45 KB (+0.2% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.1 KB (+0.23% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	32.98 KB (+0.45% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	24.07 KB (+1.04% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	210.62 KB (+0.23% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	99.48 KB (+0.49% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	71.79 KB (+1.09% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	36.08 KB (+0.43% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	68.87 KB (+0.29% 🔺)
@sentry/react - Webpack (gzipped)	22.59 KB (+1.42% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	85.29 KB (+0.23% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	49.7 KB (+0.41% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	17.4 KB (+1.79% 🔺)

[lforst]

Obligatory bikeshed: Would unstable_sentryVitePluginOptions be better?

[Lms24]

Obligatory bikeshed: Would unstable_sentryVitePluginOptions be better?

haha no worries 😅 good point, it's probably better to be explicit. I'll change it!