Ensure we support browser profiling properly

[BYK]

Full support for UI Profiling is available from version 10.27.0.
Docs are updated to include the new UI Profiling mode: https://docs.sentry.io/platforms/javascript/profiling/
The profilesSampleRate is now deprecated in favor of profileSessionSampleRate. The decisions if something should be profiled or not will now be made at SDK initialization (start of a user session on page load). And then, the whole user session will be continuously profiled.
In profileLifecycle: "manual" (default), users can start and stop profiling with Sentry.uiProfiler.startProfiler() and .stopProfiler().
In profileLifecycle: "trace" the SDK automatically profiles when there is at least one active root/segment span. This is most similar to our previous transaction-based approach.

by @s1gr1d

We should probably promote (or auto-enable) profileLifecycle: "trace" when Spotlight is enabled.

We also very likely need to address #693 first.