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
console.log();
  console.log("### Creating reference images");
  const args = ["test.js", "--masterMode"];
  if (bot) {
    args.push("--noPrompts", "--strictVerify");
  }
  if (process.argv.includes("--noChrome")) {
    args.push("--noChrome");
  }
  const testProcess = startNode(args, { cwd: TEST_DIR, stdio: "inherit" });
  testProcess.on("close", function (code) {
    done();
  });
}
gulp.task("default", function (done) {
  console.log("Available tasks:");
  const tasks = Object.keys(gulp.registry().tasks());
  for (const taskName of tasks.sort()) {
    if (taskName.endsWith("-pre")) {
      continue;
    }
    console.log("  " + taskName);
  }
  done();
});
function createBuildNumber(done) {