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
          .src(LIB_DIR + "**/*", { base: LIB_DIR })
          .pipe(gulp.dest(DIST_DIR + "lib/")),
        gulp
          .src(TYPES_DIR + "**/*", { base: TYPES_DIR })
          .pipe(gulp.dest(DIST_DIR + "types/")),
      ]);
    }
  )
);
gulp.task(
  "dist-install",
  gulp.series("dist-pre", function createDistInstall(done) {
    let distPath = DIST_DIR;
    const opts = {};
    const installPath = process.env.PDFJS_INSTALL_PATH;
    if (installPath) {
      opts.cwd = installPath;
      distPath = path.relative(installPath, distPath);
    }
    safeSpawnSync("npm", ["install", distPath], opts);
    done();
  })
);
gulp.task(
  "dist",
  gulp.series("dist-pre", function createDist(done) {
    const VERSION = getVersionJSON().version;
    console.log();
    console.log("### Committing changes");