Measure arbitrary time in the marker chart

I often need to measure the time between the start of a marker and the start of another (nested) marker, like the attached screenshot. This is very useful to (for example) measure the overhead of something that we think is cheap, but that calls an expensive function.


I've also needed to measure time in between markers. It's always possible to subtract the numbers with a bit of mental arithmetic, but it's easier to drag a ruler.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-279)