ref(replay): Streamline replay EventBuffer implementation

[mydea]

This ensures public/private annotation is correct for the event buffer implementation. It also refactors a few (internal) things in minor ways, for clarity.

For example, we used a getter which as a side effect incremented itself. It is cleaner to do this explicitly, as getters should generally be side-effect free.

This is technically a breaking change, as the exported name of the interface IEventBuffer was changed to EventBuffer. It is unlikely that a user relied on that, though.

ref #6323

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.6 KB (+0.01% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	60.69 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.37 KB (+0.02% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	54.26 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.13 KB (0%)
@sentry/browser - Webpack (minified)	65.83 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.16 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.01 KB (+0.11% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.44 KB (+0.01% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.87 KB (+0.03% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	41.7 KB (-0.1% 🔽)
@sentry/replay - Webpack (gzipped + minified)	37.21 KB (-0.07% 🔽)