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
let reason = process.env.PDFJS_UPDATE_REASON;
    
    if (typeof reason === "string") {
      const reasonParts =
        /^(See )(mozilla\/pdf\.js)@tags\/(v\d+\.\d+\.\d+)\s*$/.exec(reason);
      if (reasonParts) {
        reason =
          reasonParts[1] +
          "https://github.com/" +
          reasonParts[2] +
          "/releases/tag/" +
          reasonParts[3];
      }
    }
    const message =
      "PDF.js version " + VERSION + (reason ? " - " + reason : "");
    safeSpawnSync("git", ["add", "*"], { cwd: DIST_DIR });
    safeSpawnSync("git", ["commit", "-am", message], { cwd: DIST_DIR });
    safeSpawnSync("git", ["tag", "-a", "v" + VERSION, "-m", message], {
      cwd: DIST_DIR,
    });
    console.log();
    console.log("Done. Push with");
    console.log(
      "  cd " + DIST_DIR + "; git push --tags " + DIST_REPO_URL + " master"
    );
    console.log();
    done();
  })
);