[DevTools] Disconnect and Reconnect children of Suspense boundaries instead of Unmounting and Mounting (facebook#34089)

Stacked on facebook#34082.

This keeps the DevToolsInstance children alive inside Offscreen trees
while they're hidden. However, they're sent as unmounted to the front
end store.

This allows DevTools state to be preserved between these two states.

Such as it keeps the "suspended by" set on the SuspenseNode alive since
the children are still mounted. So now you when you resuspend, you can
see what in the children was suspended. This is useful when you're
simulating a suspense but can also be a bit misleading when something
suspended for real since it'll only show the previous suspended set and
not what is currently suspending it since that hasn't committed yet.

SuspenseNodes inside resuspended trees are now kept alive too. That way
they can contribute to the timeline even when resuspended. We can choose
whether to keep them visible in the rects while hidden or not.

In the future we'll also need to add more special cases around Activity.
Because right now if SuspenseNodes are kept alive in the Suspense tab UI
while hidden, then they're also alive inside Activity that are hidden
which maybe we don't want. Maybe simplest would be that they both
disappear from the Suspense tab UI but can be considered for the
timeline.

Another case is that when Activity goes hidden, Fiber will no longer
cause its content to suspend the parent but that's not modeled here. So
hidden Activity will show up as "suspended by" in a parent Suspense.
When they disconnect, they should really be removed from the "suspended
by" set of the parent (and perhaps be shown only on the Activity
boundary itself).

Author: sebmarkbage