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:


const outName = getTempFile("~preprocess", ".html");
  builder.preprocess(source, outName, defines);
  const out = fs.readFileSync(outName).toString();
  fs.unlinkSync(outName);
  const i = source.lastIndexOf("/");
  return createStringSource(source.substr(i + 1), `${out.trimEnd()}\n`);
}
function buildGeneric(defines, dir) {
  rimraf.sync(dir);
  return merge([
    createMainBundle(defines).pipe(gulp.dest(dir + "build")),
    createWorkerBundle(defines).pipe(gulp.dest(dir + "build")),
    createSandboxBundle(defines).pipe(gulp.dest(dir + "build")),
    createWebBundle(defines, {
      defaultPreferencesDir: defines.SKIP_BABEL
        ? "generic/"
        : "generic-legacy/",
    }).pipe(gulp.dest(dir + "web")),
    gulp.src(COMMON_WEB_FILES, { base: "web/" }).pipe(gulp.dest(dir + "web")),
    gulp.src("LICENSE").pipe(gulp.dest(dir)),
    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)