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
url: false,
      zlib: false,
    },
    format: "amd", 
    repository: {
      type: "git",
      url: DIST_REPO_URL,
    },
  };
  const bowerManifest = {
    name: DIST_NAME,
    version: VERSION,
    main: ["build/pdf.js", "build/pdf.worker.js"],
    ignore: [],
    keywords: DIST_KEYWORDS,
  };
  return [
    createStringSource("package.json", JSON.stringify(npmManifest, null, 2)),
    createStringSource("bower.json", JSON.stringify(bowerManifest, null, 2)),
  ];
}
gulp.task(
  "dist-pre",
  gulp.series(
    "generic",
    "generic-legacy",
    "components",
    "components-legacy",
    "image_decoders",
    "image_decoders-legacy",