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:


let reason = process.env.PDFJS_UPDATE_REASON;
    
    if (typeof reason === "string") {
      const reasonParts =
        /^(See )(mozilla\/pdf\.js)@tags\/(v\d+\.\d+\.\d+)\s*$/.exec(reason);
      if (reasonParts) {
        reason =
          reasonParts[1] +
          "https://github.com/" +
          reasonParts[2] +
          "/releases/tag/" +
          reasonParts[3];
      }
    }
    const message =
      "PDF.js version " + VERSION + (reason ? " - " + reason : "");
    safeSpawnSync("git", ["add", "*"], { cwd: DIST_DIR });
    safeSpawnSync("git", ["commit", "-am", message], { cwd: DIST_DIR });
    safeSpawnSync("git", ["tag", "-a", "v" + VERSION, "-m", message], {
      cwd: DIST_DIR,
    });
    console.log();
    console.log("Done. Push with");
    console.log(
      "  cd " + DIST_DIR + "; git push --tags " + DIST_REPO_URL + " master"
    );
    console.log();
    done();
  })
);