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
return merge([
    createMainBundle(defines).pipe(gulp.dest(dir + "build")),
    createWorkerBundle(defines).pipe(gulp.dest(dir + "build")),
    createSandboxBundle(defines).pipe(gulp.dest(dir + "build")),
    createWebBundle(defines, {
      defaultPreferencesDir: defines.SKIP_BABEL
        ? "minified/"
        : "minified-legacy/",
    }).pipe(gulp.dest(dir + "web")),
    createImageDecodersBundle(
      builder.merge(defines, { IMAGE_DECODERS: true })
    ).pipe(gulp.dest(dir + "image_decoders")),
    gulp.src(COMMON_WEB_FILES, { base: "web/" }).pipe(gulp.dest(dir + "web")),
    gulp
      .src(["web/locale/*/viewer.properties", "web/locale/locale.properties"], {
        base: "web/",
      })
      .pipe(gulp.dest(dir + "web")),
    createCMapBundle().pipe(gulp.dest(dir + "web/cmaps")),
    createStandardFontBundle().pipe(gulp.dest(dir + "web/standard_fonts")),
    preprocessHTML("web/viewer.html", defines).pipe(gulp.dest(dir + "web")),
    preprocessCSS("web/viewer.css", "minified", defines, true)
      .pipe(postcss([calc(), autoprefixer(AUTOPREFIXER_CONFIG)]))
      .pipe(gulp.dest(dir + "web")),
    gulp
      .src("web/compressed.tracemonkey-pldi-09.pdf")
      .pipe(gulp.dest(dir + "web")),
  ]);
}
async function parseMinified(dir) {