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(
  "mozcentraldiff",
  gulp.series(
    "mozcentral",
    "mozcentralbaseline",
    function createMozcentralDiff(done) {
      console.log();
      console.log("### Creating mozcentral diff");
      
      
      
      
      
      rimraf.sync(MOZCENTRAL_BASELINE_DIR + "*");
      gulp
        .src([BUILD_DIR + "mozcentral/**/*"])
        .pipe(gulp.dest(MOZCENTRAL_BASELINE_DIR))
        .on("end", function () {
          safeSpawnSync("git", ["add", "-A"], { cwd: MOZCENTRAL_BASELINE_DIR });
          const diff = safeSpawnSync(
            "git",
            ["diff", "--binary", "--cached", "--unified=8"],
            { cwd: MOZCENTRAL_BASELINE_DIR }
          ).stdout;
          createStringSource(MOZCENTRAL_DIFF_FILE, diff)
            .pipe(gulp.dest(BUILD_DIR))
            .on("end", function () {
              console.log(
                "Result diff can be found at " +
                  BUILD_DIR +