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:


"lib",
    "minified",
    "minified-legacy",
    "types",
    function createDist() {
      console.log();
      console.log("### Cloning baseline distribution");
      rimraf.sync(DIST_DIR);
      mkdirp.sync(DIST_DIR);
      safeSpawnSync("git", ["clone", "--depth", "1", DIST_REPO_URL, DIST_DIR]);
      console.log();
      console.log("### Overwriting all files");
      rimraf.sync(path.join(DIST_DIR, "*"));
      
      const [packageJsonSrc, bowerJsonSrc] = packageBowerJson();
      return merge([
        packageJsonSrc.pipe(gulp.dest(DIST_DIR)),
        bowerJsonSrc.pipe(gulp.dest(DIST_DIR)),
        vfs
          .src("external/dist/**/*", { base: "external/dist", stripBOM: false })
          .pipe(gulp.dest(DIST_DIR)),
        gulp.src(GENERIC_DIR + "LICENSE").pipe(gulp.dest(DIST_DIR)),
        gulp
          .src(GENERIC_DIR + "web/cmaps/**/*", { base: GENERIC_DIR + "web" })
          .pipe(gulp.dest(DIST_DIR)),
        gulp
          .src(GENERIC_DIR + "web/standard_fonts/**/*", {
            base: GENERIC_DIR + "web",
          })
          .pipe(gulp.dest(DIST_DIR)),