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:


createBuildNumber,
    "locale",
    function scriptingMinifiedLegacy() {
      const defines = builder.merge(DEFINES, {
        MINIFIED: true,
        GENERIC: true,
        SKIP_BABEL: false,
      });
      return merge([
        buildDefaultPreferences(defines, "minified-legacy/"),
        createTemporaryScriptingBundle(defines),
      ]);
    },
    function createMinifiedLegacy() {
      console.log();
      console.log("### Creating minified (legacy) viewer");
      const defines = builder.merge(DEFINES, {
        MINIFIED: true,
        GENERIC: true,
        SKIP_BABEL: false,
      });
      return buildMinified(defines, MINIFIED_LEGACY_DIR);
    },
    async function compressMinifiedLegacy(done) {
      await parseMinified(MINIFIED_LEGACY_DIR);
      done();
    }
  )
);
function preprocessDefaultPreferences(content) {