Shorten the WorkerMessageHandler class a little bit

- Use `this` in all scopes where that's possible, to avoid having to spell out `WorkerMessageHandler` everywhere.

 - Inline the `isMessagePort` helper function, since there's only a single call-site.

 - Use a static initialization block to move more code into the `WorkerMessageHandler` class itself.

src/core/worker.js

@@ -70,9 +70,23 @@ class WorkerTask {
 }
 
 class WorkerMessageHandler {
+  static {
+    // Worker thread (and not Node.js)?
+    if (
+      typeof window === "undefined" &&
+      !isNodeJS &&
+      typeof self !== "undefined" &&
+      /* isMessagePort = */
+      typeof self.postMessage === "function" &&
+      "onmessage" in self
+    ) {
+      this.initializeFromPort(self);
+    }
+  }
+
   static setup(handler, port) {
     let testMessageProcessed = false;
-    handler.on("test", function (data) {
+    handler.on("test", data => {
       if (testMessageProcessed) {
         return; // we already processed 'test' message once
       }
@@ -82,13 +96,11 @@ class WorkerMessageHandler {
       handler.send("test", data instanceof Uint8Array);
     });
 
-    handler.on("configure", function (data) {
+    handler.on("configure", data => {
       setVerbosityLevel(data.verbosity);
     });
 
-    handler.on("GetDocRequest", function (data) {
-      return WorkerMessageHandler.createDocumentHandler(data, port);
-    });
+    handler.on("GetDocRequest", data => this.createDocumentHandler(data, port));
   }
 
   static createDocumentHandler(docParams, port) {
@@ -879,25 +891,9 @@ class WorkerMessageHandler {
 
   static initializeFromPort(port) {
     const handler = new MessageHandler("worker", "main", port);
-    WorkerMessageHandler.setup(handler, port);
+    this.setup(handler, port);
     handler.send("ready", null);
   }
 }
 
-function isMessagePort(maybePort) {
-  return (
-    typeof maybePort.postMessage === "function" && "onmessage" in maybePort
-  );
-}
-
-// Worker thread (and not Node.js)?
-if (
-  typeof window === "undefined" &&
-  !isNodeJS &&
-  typeof self !== "undefined" &&
-  isMessagePort(self)
-) {
-  WorkerMessageHandler.initializeFromPort(self);
-}
-
 export { WorkerMessageHandler, WorkerTask };