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:


],
        { ignoreInitial: false },
        gulp.series("dev-sandbox")
      );
    },
    function createServer() {
      console.log();
      console.log("### Starting local server");
      const WebServer = require("./test/webserver.js").WebServer;
      const server = new WebServer();
      server.port = 8888;
      server.start();
    }
  )
);
gulp.task("clean", function (done) {
  console.log();
  console.log("### Cleaning up project builds");
  rimraf(BUILD_DIR, done);
});
gulp.task("importl10n", function (done) {
  const locales = require("./external/importL10n/locales.js");
  console.log();
  console.log("### Importing translations from mozilla-central");
  if (!fs.existsSync(L10N_DIR)) {
    fs.mkdirSync(L10N_DIR);
  }
  locales.downloadL10n(L10N_DIR, done);
});
function ghPagesPrepare() {