Show the starting time of a marker in the marker tooltip

It's currently quite difficult to determine exactly when a marker starts. We should show its timestamp in the tooltip.

Also when the start is from before the profile's start (its value would be negative), we should show it clearly. (see bugzilla [bug 1730907](https://bugzilla.mozilla.org/show_bug.cgi?id=1730907))

I don't know clearly yet how I'd like this to be displayed, we'll need to try and error a bit.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-438)