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:


gulp.task(
  "mozcentralbaseline",
  gulp.series(createBaseline, function createMozcentralBaseline(done) {
    console.log();
    console.log("### Creating mozcentral baseline environment");
    
    rimraf.sync(BASELINE_DIR + BUILD_DIR);
    const workingDirectory = path.resolve(process.cwd(), BASELINE_DIR);
    safeSpawnSync("gulp", ["mozcentral"], {
      env: process.env,
      cwd: workingDirectory,
      stdio: "inherit",
    });
    
    rimraf.sync(MOZCENTRAL_BASELINE_DIR);
    mkdirp.sync(MOZCENTRAL_BASELINE_DIR);
    gulp
      .src([BASELINE_DIR + BUILD_DIR + "mozcentral/**/*"])
      .pipe(gulp.dest(MOZCENTRAL_BASELINE_DIR))
      .on("end", function () {
        
        safeSpawnSync("git", ["init"], { cwd: MOZCENTRAL_BASELINE_DIR });
        safeSpawnSync("git", ["add", "."], { cwd: MOZCENTRAL_BASELINE_DIR });
        safeSpawnSync("git", ["commit", "-m", '"mozcentral baseline"'], {
          cwd: MOZCENTRAL_BASELINE_DIR,
        });
        done();
      });
  })
);