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(MINIFIED_LEGACY_DIR + "build/pdf.js")
          .pipe(rename("pdf.min.js"))
          .pipe(gulp.dest(DIST_DIR + "legacy/build/")),
        gulp
          .src(MINIFIED_LEGACY_DIR + "build/pdf.worker.js")
          .pipe(rename("pdf.worker.min.js"))
          .pipe(gulp.dest(DIST_DIR + "legacy/build/")),
        gulp
          .src(MINIFIED_LEGACY_DIR + "build/pdf.sandbox.js")
          .pipe(rename("pdf.sandbox.min.js"))
          .pipe(gulp.dest(DIST_DIR + "legacy/build/")),
        gulp
          .src(MINIFIED_LEGACY_DIR + "image_decoders/pdf.image_decoders.js")
          .pipe(rename("pdf.image_decoders.min.js"))
          .pipe(gulp.dest(DIST_DIR + "legacy/image_decoders/")),
        gulp
          .src(COMPONENTS_DIR + "**/*", { base: COMPONENTS_DIR })
          .pipe(gulp.dest(DIST_DIR + "web/")),
        gulp
          .src(COMPONENTS_LEGACY_DIR + "**/*", { base: COMPONENTS_LEGACY_DIR })
          .pipe(gulp.dest(DIST_DIR + "legacy/web/")),
        gulp
          .src(IMAGE_DECODERS_DIR + "**/*", { base: IMAGE_DECODERS_DIR })
          .pipe(gulp.dest(DIST_DIR + "image_decoders/")),
        gulp
          .src(IMAGE_DECODERS_LEGACY_DIR + "**/*", {
            base: IMAGE_DECODERS_LEGACY_DIR,
          })
          .pipe(gulp.dest(DIST_DIR + "legacy/image_decoders/")),