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:


.pipe(gulp.dest(GH_PAGES_DIR + "api/draft/")),
  ]);
}
gulp.task("wintersmith", function (done) {
  const wintersmith = require("wintersmith");
  const env = wintersmith("docs/config.json");
  env.build(GH_PAGES_DIR, function (error) {
    if (error) {
      done(error);
      return;
    }
    replaceInFile(
      GH_PAGES_DIR + "/getting_started/index.html",
      /STABLE_VERSION/g,
      config.stableVersion
    );
    console.log("Done building with wintersmith.");
    done();
  });
});
function ghPagesGit(done) {
  const VERSION = getVersionJSON().version;
  const reason = process.env.PDFJS_UPDATE_REASON;
  safeSpawnSync("git", ["init"], { cwd: GH_PAGES_DIR });
  safeSpawnSync("git", ["remote", "add", "origin", REPO], {
    cwd: GH_PAGES_DIR,
  });
  safeSpawnSync("git", ["add", "-A"], { cwd: GH_PAGES_DIR });
  safeSpawnSync(
    "git",