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:


return gulp
    .src(dir + "**")
    .pipe(zip(targetName))
    .pipe(gulp.dest(BUILD_DIR))
    .on("end", function () {
      console.log("Built distribution file: " + targetName);
    });
}
gulp.task(
  "publish",
  gulp.series("generic", "generic-legacy", function createPublish(done) {
    const version = JSON.parse(
      fs.readFileSync(BUILD_DIR + "version.json").toString()
    ).version;
    config.stableVersion = version;
    return merge([
      createStringSource(CONFIG_FILE, JSON.stringify(config, null, 2)).pipe(
        gulp.dest(".")
      ),
      compressPublish("pdfjs-" + version + "-dist.zip", GENERIC_DIR),
      compressPublish(
        "pdfjs-" + version + "-legacy-dist.zip",
        GENERIC_LEGACY_DIR
      ),
    ]);
  })
);
function setTestEnv(done) {