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
gulp.task(
  "mozcentralbaseline",
  gulp.series(createBaseline, function createMozcentralBaseline(done) {
    console.log();
    console.log("### Creating mozcentral baseline environment");
    
    rimraf.sync(BASELINE_DIR + BUILD_DIR);
    const workingDirectory = path.resolve(process.cwd(), BASELINE_DIR);
    safeSpawnSync("gulp", ["mozcentral"], {
      env: process.env,
      cwd: workingDirectory,
      stdio: "inherit",
    });
    
    rimraf.sync(MOZCENTRAL_BASELINE_DIR);
    mkdirp.sync(MOZCENTRAL_BASELINE_DIR);
    gulp
      .src([BASELINE_DIR + BUILD_DIR + "mozcentral/**/*"])
      .pipe(gulp.dest(MOZCENTRAL_BASELINE_DIR))
      .on("end", function () {
        
        safeSpawnSync("git", ["init"], { cwd: MOZCENTRAL_BASELINE_DIR });
        safeSpawnSync("git", ["add", "."], { cwd: MOZCENTRAL_BASELINE_DIR });
        safeSpawnSync("git", ["commit", "-m", '"mozcentral baseline"'], {
          cwd: MOZCENTRAL_BASELINE_DIR,
        });
        done();
      });
  })
);