Feature request: Add bookmarks / markers to a profile within the profiler UI

Sometimes I'm zipping around in perf.html, and there's a section of the timeline that I want to remember to come back to, or need to refer to. It might be nice to be able to add a visual marker (maybe with a label?) to that part of the timeline so it's easy for me to find and get back to.

Video editing software like Premier and Camtasia Studio have this kind of timeline marker tool.

It might even be useful if one could publish those markers and share them. This might also make educating other people about how profiles are analyzed easier.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-222)