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.dest(CHROME_BUILD_CONTENT_DIR + "build")
        ),
        createWebBundle(defines, { defaultPreferencesDir: "chromium/" }).pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")
        ),
        gulp
          .src(CHROME_WEB_FILES, { base: "web/" })
          .pipe(gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")),
        gulp
          .src(
            ["web/locale/*/viewer.properties", "web/locale/locale.properties"],
            { base: "web/" }
          )
          .pipe(gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")),
        createCMapBundle().pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "web/cmaps")
        ),
        createStandardFontBundle().pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "web/standard_fonts")
        ),
        preprocessHTML("web/viewer.html", defines).pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")
        ),
        preprocessCSS("web/viewer.css", defines)
          .pipe(
            postcss([
              postcssLogical({ preserve: true }),
              postcssDirPseudoClass(),
              autoprefixer({ overrideBrowserslist: ["Chrome >= 73"] }),
            ])