Use `globalThis` for webpack's `output.globalObject` instead of `this`. In the current configuration, `this` is used.

https://github.com/mozilla/pdf.js/blob/96b125fb729d5b0adff31b172e16cd07625552c5/gulpfile.js#L232-L233

- https://webpack.js.org/configuration/output/#outputglobalobject

As you know, `globalThis` exists for this use case. Comparing both compatibility tables of MDN and PDF.js, it seems feasible to use `globalThis`.

- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis
- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis#browser_compatibility
- https://github.com/mozilla/pdf.js/wiki/Frequently-Asked-Questions#which-browsers-are-supported

### Advantages

One of advantages of using `globalThis` is that it allows us to import `pdf.js` from modules. Let us consider the following example:

index.html:

``` html
<script src="main.js" type="module"></script>
```

main.js: 
```js
import './pdf.js'
console.log(pdfjsLib)
```

With the current `this` configuration, we see an error since the top level [`this` will be undefined in Modules](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/globalThis#description).

```
Uncaught TypeError: root is undefined
```

- Web browser and its version: Firefox 100.0
- Operating system and its version: macOS Big Sur 11.6.5
- PDF.js version: the current master 96b125fb729d5b0adff31b172e16cd07625552c5

With the `globalThis` configuration, we see no error:


if (process.argv.includes("--noChrome") || forceNoChrome) {
    args.push("--noChrome");
  }
  const testProcess = startNode(args, { cwd: TEST_DIR, stdio: "inherit" });
  testProcess.on("close", function (code) {
    done();
  });
}
gulp.task("default", function (done) {
  console.log("Available tasks:");
  const tasks = Object.keys(gulp.registry().tasks());
  for (const taskName of tasks.sort()) {
    if (taskName.endsWith("-pre")) {
      continue;
    }
    console.log("  " + taskName);
  }
  done();
});
function createBuildNumber(done) {
  console.log();
  console.log("### Getting extension build number");
  exec(
    "git log --format=oneline " + config.baseVersion + "..",
    function (err, stdout, stderr) {
      let buildNumber = 0;
      if (!err) {
        
        buildNumber = stdout ? stdout.match(/\n/g).length : 0;
      } else {