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 scriptingAMDName = "pdfjs-dist/build/pdf.scripting";
  const scriptingOutputName = "pdf.scripting.js";
  const scriptingFileConfig = createWebpackConfig(
    defines,
    {
      filename: scriptingOutputName,
      library: scriptingAMDName,
      libraryTarget: "umd",
      umdNamedDefine: true,
    },
    extraOptions
  );
  return gulp
    .src("./src/pdf.scripting.js")
    .pipe(webpack2Stream(scriptingFileConfig))
    .pipe(replaceWebpackRequire())
    .pipe(replaceNonWebpackImport())
    .pipe(
      replace(
        'root["' + scriptingAMDName + '"] = factory()',
        "root.pdfjsScripting = factory()"
      )
    );
}
function createSandboxExternal(defines) {