Async CLI watch callbacks

[bjufre]

While trying to implement a small custom little wrapper around Remix, I found some limitations with the cli API. While we can manage with the build and some sync code, it becomes more difficult or inconvenient when dealing with watch or dev.
I know that right now, the team doesn't want to open the compiler to the outside in order to avoid possible problems or complex logic creeping into the codebase. Nonetheless, I really think that a good compromise would be to allow the WatchCallbacks to be async in order to allow for minimal interop with the Remix compiler.

type WatchCallbacks = {
  onRebuildStart?(): Promise<void>;
  onInitialBuild?(): Promise<void>;
};

At the same time, it might be worth it, to allow such callbacks to be passed into the dev command as well 🙏