workers: basic event loop (#3828)

* establish basic event loop for workers
* make "self.close()" inside worker
* remove "runWorkerMessageLoop() - instead manually call global function 
  in Rust when message arrives. This is done in preparation for structured clone
* refactor "WorkerChannel" and use distinct structs for internal 
  and external channels;  "WorkerChannelsInternal" and "WorkerHandle"
* move "State.worker_channels_internal" to "Worker.internal_channels"
* add "WorkerEvent" enum for child->host communication; 
  currently "Message(Buf)" and  "Error(ErrBox)" variants are supported
* add tests for nested workers
* add tests for worker throwing error on startup

cli/js/lib.deno.worker.d.ts

@@ -37,7 +37,6 @@ declare const postMessage: typeof __workerMain.postMessage;
 declare namespace __workerMain {
   export let onmessage: (e: { data: any }) => void;
   export function postMessage(data: any): void;
-  export function getMessage(): Promise<any>;
   export function close(): void;
   export const name: string;
 }