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:


)
          .pipe(gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")),
        gulp.src("LICENSE").pipe(gulp.dest(CHROME_BUILD_DIR)),
        gulp
          .src("extensions/chromium/manifest.json")
          .pipe(replace(/\bPDFJSSCRIPT_VERSION\b/g, version))
          .pipe(gulp.dest(CHROME_BUILD_DIR)),
        gulp
          .src(
            [
              "extensions/chromium/**/*.{html,js,css,png}",
              "extensions/chromium/preferences_schema.json",
            ],
            { base: "extensions/chromium/" }
          )
          .pipe(gulp.dest(CHROME_BUILD_DIR)),
      ]);
    }
  )
);
gulp.task("jsdoc", function (done) {
  console.log();
  console.log("### Generating documentation (JSDoc)");
  const JSDOC_FILES = ["src/display/api.js"];
  rimraf(JSDOC_BUILD_DIR, function () {
    mkdirp(JSDOC_BUILD_DIR).then(function () {
      const command =
        '"node_modules/.bin/jsdoc" -d ' +
        JSDOC_BUILD_DIR +
        " " +