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
],
        { ignoreInitial: false },
        gulp.series("dev-sandbox")
      );
    },
    function createServer() {
      console.log();
      console.log("### Starting local server");
      const WebServer = require("./test/webserver.js").WebServer;
      const server = new WebServer();
      server.port = 8888;
      server.start();
    }
  )
);
gulp.task("clean", function (done) {
  console.log();
  console.log("### Cleaning up project builds");
  rimraf(BUILD_DIR, done);
});
gulp.task("importl10n", function (done) {
  const locales = require("./external/importL10n/locales.js");
  console.log();
  console.log("### Importing translations from mozilla-central");
  if (!fs.existsSync(L10N_DIR)) {
    fs.mkdirSync(L10N_DIR);
  }
  locales.downloadL10n(L10N_DIR, done);
});
function ghPagesPrepare() {