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:


function (error2) {
        if (error2) {
          done(new Error("Baseline commit checkout failed."));
          return;
        }
        console.log('Baseline commit "' + baselineCommit + '" checked out.');
        done();
      }
    );
  });
}
gulp.task(
  "unittestcli",
  gulp.series(setTestEnv, "lib-legacy", function runUnitTestCli(done) {
    const options = [
      "node_modules/jasmine/bin/jasmine",
      "JASMINE_CONFIG_PATH=test/unit/clitests.json",
    ];
    const jasmineProcess = startNode(options, { stdio: "inherit" });
    jasmineProcess.on("close", function (code) {
      if (code !== 0) {
        done(new Error("Unit tests failed."));
        return;
      }
      done();
    });
  })
);
gulp.task("lint", function (done) {
  console.log();