Scaffolding for concurrentRouterQueue flag (#97413)
Sets up the scaffolding for a rewrite of the client router's queue.
I've named the new module concurrent-router-queue.ts and the existing
module sequential-router-queue.ts, since the main goal of the new one is
to improve the router's integration with React's concurrent rendering
model. The experimental flag is likewise named `concurrentRouterQueue`.
There's no implementation yet; all router operations throw an error when
the flag is enabled.
The main entry point into the router is navigator.ts. The flag swaps
between implementations at the module level to ensure the inactive code
is eliminated from the final bundle.
