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:


MOZCENTRAL_DIFF_FILE
              );
              done();
            });
        });
    }
  )
);
gulp.task("externaltest", function (done) {
  console.log();
  console.log("### Running test-fixtures.js");
  safeSpawnSync("node", ["external/builder/test-fixtures.js"], {
    stdio: "inherit",
  });
  console.log();
  console.log("### Running test-fixtures_esprima.js");
  safeSpawnSync("node", ["external/builder/test-fixtures_esprima.js"], {
    stdio: "inherit",
  });
  done();
});
gulp.task(
  "ci-test",
  gulp.series(
    gulp.parallel("lint", "externaltest", "unittestcli"),
    "lint-chromium",
    "typestest"
  )
);