Consider hovering strategy for JS Tracer self-time view (and regular view)

Follow-up to #1422.

Currently a hovered item is a reference into the JS tracer data structure. However, the self-time view potentially breaks up a single event into many smaller chunks. This makes it really awkward to hover tiny boxes, and the entire row lights up, even if it only makes up a small percentage of the time. The current strategy mitigates this by only darkening a color of the highlighted item if its width is greater than 1.

So, what is useful when hovering on the chart? Oftentimes an individual pixel can contain hundreds if not thousands of individual events. We should discuss and spec out the right behavior here.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-96)