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(
          "This is not a Git repository; using default build number."
        );
      }
      console.log("Extension build number: " + buildNumber);
      const version = config.versionPrefix + buildNumber;
      exec('git log --format="%h" -n 1', function (err2, stdout2, stderr2) {
        let buildCommit = "";
        if (!err2) {
          buildCommit = stdout2.replace("\n", "");
        }
        createStringSource(
          "version.json",
          JSON.stringify(
            {
              version,
              build: buildNumber,
              commit: buildCommit,
            },
            null,
            2
          )
        )
          .pipe(gulp.dest(BUILD_DIR))
          .on("end", done);
      });
    }
  );
}
function buildDefaultPreferences(defines, dir) {