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:


console.log("Skipping invalid locale: " + locale);
      continue;
    }
    mkdirp.sync(VIEWER_LOCALE_OUTPUT + "/" + locale);
    locales.push(locale);
    if (checkFile(dirPath + "/viewer.properties")) {
      viewerOutput +=
        "[" +
        locale +
        "]\n" +
        "@import url(" +
        locale +
        "/viewer.properties)\n\n";
    }
  }
  return merge([
    createStringSource("locale.properties", viewerOutput).pipe(
      gulp.dest(VIEWER_LOCALE_OUTPUT)
    ),
    gulp
      .src(L10N_DIR + "/{" + locales.join(",") + "}/viewer.properties", {
        base: L10N_DIR,
      })
      .pipe(gulp.dest(VIEWER_LOCALE_OUTPUT)),
  ]);
});
gulp.task("cmaps", function (done) {
  const CMAP_INPUT = "external/cmaps";
  const VIEWER_CMAP_OUTPUT = "external/bcmaps";
  console.log();