Add web worker JS API (#1993)

* Refactored the way worker polling is scheduled and errors are handled.
* Share the worker future as a Shared

tools/ts_library_builder/main.ts

@@ -46,6 +46,7 @@ buildRuntimeLib({
     "js/deno.ts",
     "js/globals.ts"
   ],
+  declareAsLet: ["onmessage"],
   outFile,
   silent
 });