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
rimraf.sync(MOZCENTRAL_DIR);
      const versionJSON = getVersionJSON();
      const version = versionJSON.version,
        commit = versionJSON.commit;
      
      
      const MOZCENTRAL_COMMON_WEB_FILES = [
        ...COMMON_WEB_FILES,
        "!web/images/*.cur",
      ];
      return merge([
        createMainBundle(defines).pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "build")
        ),
        createScriptingBundle(defines).pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "build")
        ),
        createSandboxExternal(defines).pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "build")
        ),
        createWorkerBundle(defines).pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "build")
        ),
        createWebBundle(defines, { defaultPreferencesDir: "mozcentral/" }).pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")
        ),
        gulp
          .src(MOZCENTRAL_COMMON_WEB_FILES, { base: "web/" })
          .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")),