Add previous_trace span link to browserTracingIntegration root spans

[Lms24]

Description

This task involves using the span link APIs introduced in #14991 to store a link to the local root span of the previous trace.

The rough implementation idea is outlined in the RFC but I'll note down the most important and some additional aspects:

• Find a place to store the previous trace root span context (rootSpan.spanContext()). This could be on the isolation scope for example but I'll leave the decision happily to implementors
• On span start, check if a valid previous trace root span context is stored and add a link to it
• Right after, replace the previous trace root span context with the current root span context
• A previous trace root span context is valid as long as it is not older than 15 minutes  1 hour (1h was chosen as a compromise of longevity and an efficient querying time window)

WIP; ref getsentry/rfcs#141

[github-actions]

A PR closing this issue has just been released 🚀

This issue was referenced by PR #15569, which was included in the 9.9.0 release.