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:


setTestEnv,
    "generic",
    "types",
    function createTypesTest() {
      const [packageJsonSrc] = packageBowerJson();
      return merge([
        packageJsonSrc.pipe(gulp.dest(TYPESTEST_DIR)),
        gulp
          .src([
            GENERIC_DIR + "build/pdf.js",
            GENERIC_DIR + "build/pdf.worker.js",
            SRC_DIR + "pdf.worker.entry.js",
          ])
          .pipe(gulp.dest(TYPESTEST_DIR + "build/")),
        gulp
          .src(TYPES_DIR + "**/*", { base: TYPES_DIR })
          .pipe(gulp.dest(TYPESTEST_DIR + "types/")),
      ]);
    },
    function runTypesTest(done) {
      exec('"node_modules/.bin/tsc" -p test/types', function (err, stdout) {
        if (err) {
          console.log(`Couldn't compile TypeScript test: ${stdout}`);
        }
        done(err);
      });
    }
  )
);
function createBaseline(done) {