Make it easier to identify samples where we were _only_ running script

This is distinct from the "JS Only" view in the Tree View, since that feature shows us stacks where JS is _involved_. What I'm requesting here is slightly deeper - what I'm hoping for is something that highlights samples where the deepest part of the stack is still JS, and that the JS isn't calling into platform-y things.

I think this would help us identify things like "this page is just running super expensive algorithms in JS".

/cc jrmuizel

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-696)