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:


? "minified/"
        : "minified-legacy/",
    }).pipe(gulp.dest(dir + "web")),
    createImageDecodersBundle(
      builder.merge(defines, { IMAGE_DECODERS: true })
    ).pipe(gulp.dest(dir + "image_decoders")),
    gulp.src(COMMON_WEB_FILES, { base: "web/" }).pipe(gulp.dest(dir + "web")),
    gulp
      .src(["web/locale/*/viewer.properties", "web/locale/locale.properties"], {
        base: "web/",
      })
      .pipe(gulp.dest(dir + "web")),
    createCMapBundle().pipe(gulp.dest(dir + "web/cmaps")),
    createStandardFontBundle().pipe(gulp.dest(dir + "web/standard_fonts")),
    preprocessHTML("web/viewer.html", defines).pipe(gulp.dest(dir + "web")),
    preprocessCSS("web/viewer.css", defines)
      .pipe(
        postcss([
          postcssLogical({ preserve: true }),
          postcssDirPseudoClass(),
          autoprefixer(AUTOPREFIXER_CONFIG),
        ])
      )
      .pipe(gulp.dest(dir + "web")),
    gulp
      .src("web/compressed.tracemonkey-pldi-09.pdf")
      .pipe(gulp.dest(dir + "web")),
  ]);
}
async function parseMinified(dir) {