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
args.push("--manifestFile=" + PDF_TEST);
        break;
      case "unit":
        args.push("--unitTest");
        break;
      case "font":
        args.push("--fontTest");
        break;
      case "integration":
        args.push("--integration");
        break;
      default:
        this.emit("error", new Error("Unknown name: " + testsName));
        return null;
    }
    if (bot) {
      args.push("--strictVerify");
    }
    if (process.argv.includes("--noChrome")) {
      args.push("--noChrome");
    }
    const testProcess = startNode(args, { cwd: TEST_DIR, stdio: "inherit" });
    testProcess.on("close", function (code) {
      source.push(null);
    });
    return undefined;
  };
  return source;
}
function makeRef(done, bot) {