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:


const COMPONENTS_DIR = BUILD_DIR + "components/";
const COMPONENTS_LEGACY_DIR = BUILD_DIR + "components-legacy/";
const IMAGE_DECODERS_DIR = BUILD_DIR + "image_decoders/";
const IMAGE_DECODERS_LEGACY_DIR = BUILD_DIR + "image_decoders-legacy/";
const DEFAULT_PREFERENCES_DIR = BUILD_DIR + "default_preferences/";
const MINIFIED_DIR = BUILD_DIR + "minified/";
const MINIFIED_LEGACY_DIR = BUILD_DIR + "minified-legacy/";
const JSDOC_BUILD_DIR = BUILD_DIR + "jsdoc/";
const GH_PAGES_DIR = BUILD_DIR + "gh-pages/";
const SRC_DIR = "src/";
const LIB_DIR = BUILD_DIR + "lib/";
const DIST_DIR = BUILD_DIR + "dist/";
const TYPES_DIR = BUILD_DIR + "types/";
const TMP_DIR = BUILD_DIR + "tmp/";
const TYPESTEST_DIR = BUILD_DIR + "typestest/";
const COMMON_WEB_FILES = [
  "web/images/*.{png,svg,gif}",
  "web/debugger.{css,js}",
];
const MOZCENTRAL_DIFF_FILE = "mozcentral.diff";
const REPO = "git@github.com:mozilla/pdf.js.git";
const DIST_REPO_URL = "https://github.com/mozilla/pdfjs-dist";
const builder = require("./external/builder/builder.js");
const CONFIG_FILE = "pdfjs.config";
const config = JSON.parse(fs.readFileSync(CONFIG_FILE).toString());
const AUTOPREFIXER_CONFIG = {
  overrideBrowserslist: [
    "last 2 versions",
    "Chrome >= 73",
    "Firefox ESR",