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
const preprocessor2 = require("./external/builder/preprocessor2.js");
  const licenseHeader = fs.readFileSync("./src/license_header.js").toString();
  const ctx = {
    saveComments: false,
    defines,
  };
  return gulp.src("./src/pdf.sandbox.external.js").pipe(
    transform("utf8", content => {
      content = preprocessor2.preprocessPDFJSCode(ctx, content);
      return `${licenseHeader}\n${content}`;
    })
  );
}
function createTemporaryScriptingBundle(defines, extraOptions = undefined) {
  return createScriptingBundle(defines, {
    disableVersionInfo: !!(extraOptions && extraOptions.disableVersionInfo),
    disableSourceMaps: true,
    disableLicenseHeader: true,
  }).pipe(gulp.dest(TMP_DIR));
}
function createSandboxBundle(defines, extraOptions = undefined) {