dillon1000/react

[DevTools] Compute environment names for the timeline (#34892)

Stacked on #34885.

This refactors the timeline to store not just an id but a complex object
for each step. This will later represent a group of boundaries.

Each timeline step is assigned an environment name. We pick the last
environment name (assumed to have resolved last) from the union of the
parent and child environment names. I.e. a child step is considered to
be blocked by the parent so if a child isn't blocked on any environment
name it still gets marked as the parent's environment name.

In a follow up, I'd like to reorder the document order timeline based on
environment names to favor loading everything in one environment before
the next.