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:


console.log("### Linting JS/CSS files");
  
  const esLintOptions = [
    "node_modules/eslint/bin/eslint",
    "--ext",
    ".js,.jsm,.json",
    ".",
    "--report-unused-disable-directives",
  ];
  if (process.argv.includes("--fix")) {
    esLintOptions.push("--fix");
  }
  const styleLintOptions = [
    "node_modules/stylelint/bin/stylelint",
    "**/*.css",
    "--report-needless-disables",
  ];
  if (process.argv.includes("--fix")) {
    styleLintOptions.push("--fix");
  }
  const esLintProcess = startNode(esLintOptions, { stdio: "inherit" });
  esLintProcess.on("close", function (esLintCode) {
    if (esLintCode !== 0) {
      done(new Error("ESLint failed."));
      return;
    }
    const styleLintProcess = startNode(styleLintOptions, { stdio: "inherit" });
    styleLintProcess.on("close", function (styleLintCode) {
      if (styleLintCode !== 0) {
        done(new Error("Stylelint failed."));