Data transform: Collapse all functions in a given namespace

This is similar to https://github.com/devtools-html/perf.html/issues/238 but instead of collapsing/charging time spent "in JS" it would be nice to do it for everything in a given namespace or path, For example, one could combine the fix for https://github.com/devtools-html/perf.html/issues/238 and collapsing/charging everything in the "js" and "JS" namespaces, or in the "js/" directory, to cleanly separate out the "JS" time from "other Gecko" time.

If the condition for collapsing/charging could be negated, the collapsing could also be done the other way, to effectively collapse together all the non-JS Gecko bits and focus on what's going on with the JS side.

cc gregtatum

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-246)