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:


},
        ],
        babelPluginReplaceNonWebpackImports,
      ],
    }).code;
    const removeCjsSrc =
      /^(var\s+\w+\s*=\s*(_interopRequireDefault\()?require\(".*?)(?:\/src)(\/[^"]*"\)\)?;)$/gm;
    content = content.replace(removeCjsSrc, (all, prefix, interop, suffix) => {
      return prefix + suffix;
    });
    return licenseHeaderLibre + content;
  }
  const babel = require("@babel/core");
  const ctx = {
    rootPath: __dirname,
    saveComments: false,
    defines: bundleDefines,
    map: {
      "pdfjs-lib": "../pdf",
    },
  };
  const licenseHeaderLibre = fs
    .readFileSync("./src/license_header_libre.js")
    .toString();
  const preprocessor2 = require("./external/builder/preprocessor2.js");
  return inputStream
    .pipe(transform("utf8", preprocess))
    .pipe(gulp.dest(outputDir));
}
function buildLib(defines, dir) {