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:


options: {
            rootPath: __dirname,
            saveComments: false,
            defines: bundleDefines,
          },
        },
      ],
    },
    
    
    node: false,
  };
}
function webpack2Stream(webpackConfig) {
  
  return webpackStream(webpackConfig, webpack2);
}
function getVersionJSON() {
  return JSON.parse(fs.readFileSync(BUILD_DIR + "version.json").toString());
}
function checkChromePreferencesFile(chromePrefsPath, webPrefs) {
  const chromePrefs = JSON.parse(fs.readFileSync(chromePrefsPath).toString());
  const chromePrefsKeys = Object.keys(chromePrefs.properties).filter(key => {
    const description = chromePrefs.properties[key].description;
    
    
    
    return !description || !description.startsWith("DEPRECATED.");
  });
  chromePrefsKeys.sort();