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("### Building cmaps");
  
  if (!checkFile(CMAP_INPUT + "/UniJIS-UCS2-H")) {
    console.log("./external/cmaps has no cmap files, download them from:");
    console.log("  https://github.com/adobe-type-tools/cmap-resources");
    throw new Error("cmap files were not found");
  }
  
  fs.readdirSync(VIEWER_CMAP_OUTPUT).forEach(function (file) {
    if (/\.bcmap$/i.test(file)) {
      fs.unlinkSync(VIEWER_CMAP_OUTPUT + "/" + file);
    }
  });
  const compressCmaps =
    require("./external/cmapscompress/compress.js").compressCmaps;
  compressCmaps(CMAP_INPUT, VIEWER_CMAP_OUTPUT, true);
  done();
});
function preprocessCSS(source, defines) {
  const outName = getTempFile("~preprocess", ".css");
  builder.preprocess(source, outName, defines);
  let out = fs.readFileSync(outName).toString();
  fs.unlinkSync(outName);
  
  const reg = /\n\/\* Copyright(.|\n)*?Mozilla Foundation(.|\n)*?\*\//g;
  out = out.replace(reg, "");
  const i = source.lastIndexOf("/");
  return createStringSource(source.substr(i + 1), out);
}
function preprocessHTML(source, defines) {