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 postcssDirPseudoClass = require("postcss-dir-pseudo-class");
const postcssLogical = require("postcss-logical");
const fs = require("fs");
const gulp = require("gulp");
const postcss = require("gulp-postcss");
const rename = require("gulp-rename");
const replace = require("gulp-replace");
const mkdirp = require("mkdirp");
const path = require("path");
const rimraf = require("rimraf");
const stream = require("stream");
const exec = require("child_process").exec;
const spawn = require("child_process").spawn;
const spawnSync = require("child_process").spawnSync;
const streamqueue = require("streamqueue");
const merge = require("merge-stream");
const zip = require("gulp-zip");
const webpack2 = require("webpack");
const webpackStream = require("webpack-stream");
const Vinyl = require("vinyl");
const vfs = require("vinyl-fs");
const through = require("through2");
const BUILD_DIR = "build/";
const L10N_DIR = "l10n/";
const TEST_DIR = "test/";
const EXTENSION_SRC_DIR = "extensions/";
const BASELINE_DIR = BUILD_DIR + "baseline/";
const MOZCENTRAL_BASELINE_DIR = BUILD_DIR + "mozcentral.baseline/";
const GENERIC_DIR = BUILD_DIR + "generic/";
const GENERIC_LEGACY_DIR = BUILD_DIR + "generic-legacy/";