[DevTools] Keep a Suspense Tree Parellel to the Instance tree in the Backend (facebook#34050)

This keeps a data structure of Suspense boundaries and the root which
can keep track which boundaries might participate in a loading sequence
and everything that suspends them. This will power the Suspense tab.

Now when you select a `<Suspense>` boundary the "suspended by" section
shows the whole boundary instead of just that component.

In the future, we'll likely need to add "Activity" boundaries to this
tree as well, so that we can track what suspended the root of an
Activity when filtering a subtree. Similar to how the root SuspenseNode
now tracks suspending at the root. Maybe it's ok to just traverse to
collect this information on-demand when you select one though since this
doesn't contribute to the deduping.

We'll also need to add implicit Suspense boundaries for the rows of a
SuspenseList with `tail=hidden/collapsed`.

Author: sebmarkbage