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:


done();
    }
  )
);
gulp.task("dev-css", function createDevCSS() {
  console.log();
  console.log("### Building development CSS");
  const defines = builder.merge(DEFINES, { GENERIC: true, TESTING: true });
  const cssDir = BUILD_DIR + "dev-css/";
  return merge([
    gulp.src("web/images/*", { base: "web/" }).pipe(gulp.dest(cssDir)),
    preprocessCSS("web/viewer.css", defines)
      .pipe(
        postcss([
          postcssLogical({ preserve: true }),
          postcssDirPseudoClass(),
          autoprefixer({ overrideBrowserslist: ["last 2 versions"] }),
        ])
      )
      .pipe(gulp.dest(cssDir)),
  ]);
});
gulp.task(
  "dev-sandbox",
  gulp.series(
    function scriptingDevSandbox() {
      const defines = builder.merge(DEFINES, { GENERIC: true, TESTING: true });
      return createTemporaryScriptingBundle(defines, {
        disableVersionInfo: true,
      });