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
],
      { base: "src/" }
    ),
    gulp.src(
      ["examples/node/domstubs.js", "web/*.js", "!web/{pdfjs,viewer}.js"],
      { base: "." }
    ),
    gulp.src("test/unit/*.js", { base: "." }),
  ]);
  return buildLibHelper(bundleDefines, inputStream, dir);
}
gulp.task(
  "lib",
  gulp.series(
    createBuildNumber,
    function scriptingLib() {
      const defines = builder.merge(DEFINES, { GENERIC: true, LIB: true });
      return merge([
        buildDefaultPreferences(defines, "lib/"),
        createTemporaryScriptingBundle(defines),
      ]);
    },
    function createLib() {
      const defines = builder.merge(DEFINES, { GENERIC: true, LIB: true });
      return merge([
        buildLib(defines, "build/lib/"),
        createSandboxBundle(defines).pipe(gulp.dest("build/lib/")),
      ]);
    }
  )