Improved threaded stacktrace UI

[malwilley]

Problem

The current UI for threaded stack traces is a bit disjointed and the UX when using it is sub-par. Some feedback we've received:

• Main thread is not identified in the selector: Provide "is main thread" attribute team-mobile#198
• Scrolling through threads to find what you're looking for is quite difficult: Add next and previous thread buttons #20746
• It is difficult to know from the selector which threads actually have interesting stack traces (many threads have empty stack traces or stack traces with no in-app frames)

Internal: see here for an example issue with threaded stack trace

Solution

TBD

[getsantry]

Routing to @getsentry/product-owners-issues for triage ⏲️