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:


"Safari >= 12.1",
    "> 1%",
    "not IE > 0",
    "not dead",
  ],
};
const DEFINES = Object.freeze({
  PRODUCTION: true,
  SKIP_BABEL: true,
  TESTING: undefined,
  
  GENERIC: false,
  MOZCENTRAL: false,
  CHROME: false,
  MINIFIED: false,
  COMPONENTS: false,
  LIB: false,
  IMAGE_DECODERS: false,
});
function transform(charEncoding, transformFunction) {
  return through.obj(function (vinylFile, enc, done) {
    const transformedFile = vinylFile.clone();
    transformedFile.contents = Buffer.from(
      transformFunction(transformedFile.contents),
      charEncoding
    );
    done(null, transformedFile);
  });
}
function safeSpawnSync(command, parameters, options) {