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
console.log();
  console.log("### Creating web site");
  rimraf.sync(GH_PAGES_DIR);
  
  return merge([
    vfs
      .src(GENERIC_DIR + "**/*", { base: GENERIC_DIR, stripBOM: false })
      .pipe(gulp.dest(GH_PAGES_DIR)),
    vfs
      .src(GENERIC_LEGACY_DIR + "**/*", {
        base: GENERIC_LEGACY_DIR,
        stripBOM: false,
      })
      .pipe(gulp.dest(GH_PAGES_DIR + "legacy/")),
    gulp
      .src(JSDOC_BUILD_DIR + "**/*", { base: JSDOC_BUILD_DIR })
      .pipe(gulp.dest(GH_PAGES_DIR + "api/draft/")),
  ]);
}
gulp.task("wintersmith", function (done) {
  const wintersmith = require("wintersmith");
  const env = wintersmith("docs/config.json");
  env.build(GH_PAGES_DIR, function (error) {
    if (error) {
      done(error);
      return;