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
plugins: babelPlugins,
          },
        },
        {
          loader: path.join(__dirname, "external/webpack/pdfjsdev-loader.js"),
          options: {
            rootPath: __dirname,
            saveComments: false,
            defines: bundleDefines,
          },
        },
      ],
    },
    
    
    node: false,
  };
}
function webpack2Stream(webpackConfig) {
  
  return webpackStream(webpackConfig, webpack2);
}
function getVersionJSON() {
  return JSON.parse(fs.readFileSync(BUILD_DIR + "version.json").toString());
}
function checkChromePreferencesFile(chromePrefsPath, webPrefs) {
  const chromePrefs = JSON.parse(fs.readFileSync(chromePrefsPath).toString());
  const chromePrefsKeys = Object.keys(chromePrefs.properties).filter(key => {
    const description = chromePrefs.properties[key].description;