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:


mkdirp.sync(BUILD_DIR + "tmp/");
  const bytes = require("crypto").randomBytes(6).toString("hex");
  const filePath = BUILD_DIR + "tmp/" + prefix + bytes + suffix;
  fs.writeFileSync(filePath, "");
  return filePath;
}
function createTestSource(testsName, { bot = false, xfaOnly = false } = {}) {
  const source = stream.Readable({ objectMode: true });
  source._read = function () {
    console.log();
    console.log("### Running " + testsName + " tests");
    const PDF_TEST = process.env.PDF_TEST || "test_manifest.json";
    let forceNoChrome = false;
    const args = ["test.js"];
    switch (testsName) {
      case "browser":
        if (!bot) {
          args.push("--reftest");
        } else {
          const os = process.env.OS;
          if (/windows/i.test(os)) {
            
            
            forceNoChrome = true;
          }
        }
        if (xfaOnly) {
          args.push("--xfaOnly");
        }