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:


gulp
          .src(LIB_DIR + "**/*", { base: LIB_DIR })
          .pipe(gulp.dest(DIST_DIR + "lib/")),
        gulp
          .src(TYPES_DIR + "**/*", { base: TYPES_DIR })
          .pipe(gulp.dest(DIST_DIR + "types/")),
      ]);
    }
  )
);
gulp.task(
  "dist-install",
  gulp.series("dist-pre", function createDistInstall(done) {
    let distPath = DIST_DIR;
    const opts = {};
    const installPath = process.env.PDFJS_INSTALL_PATH;
    if (installPath) {
      opts.cwd = installPath;
      distPath = path.relative(installPath, distPath);
    }
    safeSpawnSync("npm", ["install", distPath], opts);
    done();
  })
);
gulp.task(
  "dist",
  gulp.series("dist-pre", function createDist(done) {
    const VERSION = getVersionJSON().version;
    console.log();
    console.log("### Committing changes");