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
console.log();
  console.log("### Building default preferences");
  const bundleDefines = builder.merge(defines, {
    LIB: true,
    SKIP_BABEL: false,
    BUNDLE_VERSION: 0, 
    BUNDLE_BUILD: 0, 
    TESTING: defines.TESTING || process.env.TESTING === "true",
  });
  const inputStream = merge([
    gulp.src(["web/app_options.js"], {
      base: ".",
    }),
  ]);
  return buildLibHelper(
    bundleDefines,
    inputStream,
    DEFAULT_PREFERENCES_DIR + dir
  );
}
function getDefaultPreferences(dir) {