Support sharing context objects between concurrent renderers (#12779)

* Support concurrent primary and secondary renderers.

As a workaround to support multiple concurrent renderers, we categorize
some renderers as primary and others as secondary. We only expect
there to be two concurrent renderers at most: React Native (primary) and
Fabric (secondary); React DOM (primary) and React ART (secondary).
Secondary renderers store their context values on separate fields.

* Add back concurrent renderer warning

Only warn for two concurrent primary or two concurrent secondary renderers.

* Change "_secondary" suffix to "2"

#EveryBitCounts

src/ReactTestRenderer.js

@@ -226,6 +226,8 @@ const TestRenderer = ReactFiberReconciler({
   // Even after the reconciler has initialized and read host config values.
   now: () => nowImplementation(),
 
+  isPrimaryRenderer: true,
+
   mutation: {
     commitUpdate(
       instance: Instance,