Related to  #11474 and #12815.

The following should be added to `package.json` of `pdfjs-dist` for `worker-loader`.
```
  "peerDependenciesMeta": {
    "worker-loader": {
      "optional": true
    }
  }
```

npm v7 installs `peerDependencies` by default. We can see that calling `npm install pdfjs-dist` in an empty directory will install `webpack` and others in `node_modules`.

> In npm versions 3 through 6, peerDependencies were not automatically installed, and would raise a warning if an invalid version of the peer dependency was found in the tree. As of npm v7, peerDependencies are installed by default.

- https://docs.npmjs.com/cli/v7/configuring-npm/package-json#peerdependencies
- https://github.blog/2020-10-13-presenting-v7-0-0-of-the-npm-cli/

We can make `worker-loader` optional with `peerDependenciesMeta`.

- https://docs.npmjs.com/cli/v7/configuring-npm/package-json#peerdependenciesmeta
- https://github.com/npm/rfcs/issues/221
return (
    licenseHeader +
    "\n" +
    GLOBALS +
    "\n" +
    MODIFICATION_WARNING +
    "\n" +
    content +
    "\n"
  );
}
gulp.task(
  "mozcentral",
  gulp.series(
    createBuildNumber,
    function scriptingMozcentral() {
      const defines = builder.merge(DEFINES, { MOZCENTRAL: true });
      return buildDefaultPreferences(defines, "mozcentral/");
    },
    function createMozcentral() {
      console.log();
      console.log("### Building mozilla-central extension");
      const defines = builder.merge(DEFINES, { MOZCENTRAL: true });
      const MOZCENTRAL_DIR = BUILD_DIR + "mozcentral/",
        MOZCENTRAL_EXTENSION_DIR = MOZCENTRAL_DIR + "browser/extensions/pdfjs/",
        MOZCENTRAL_CONTENT_DIR = MOZCENTRAL_EXTENSION_DIR + "content/",
        FIREFOX_EXTENSION_DIR = "extensions/firefox/",
        MOZCENTRAL_L10N_DIR =
          MOZCENTRAL_DIR + "browser/locales/en-US/pdfviewer/",
        FIREFOX_CONTENT_DIR = EXTENSION_SRC_DIR + "/firefox/content/";