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
"lib",
    "minified",
    "minified-legacy",
    "types",
    function createDist() {
      console.log();
      console.log("### Cloning baseline distribution");
      rimraf.sync(DIST_DIR);
      mkdirp.sync(DIST_DIR);
      safeSpawnSync("git", ["clone", "--depth", "1", DIST_REPO_URL, DIST_DIR]);
      console.log();
      console.log("### Overwriting all files");
      rimraf.sync(path.join(DIST_DIR, "*"));
      
      const [packageJsonSrc, bowerJsonSrc] = packageBowerJson();
      return merge([
        packageJsonSrc.pipe(gulp.dest(DIST_DIR)),
        bowerJsonSrc.pipe(gulp.dest(DIST_DIR)),
        vfs
          .src("external/dist/**/*", { base: "external/dist", stripBOM: false })
          .pipe(gulp.dest(DIST_DIR)),
        gulp.src(GENERIC_DIR + "LICENSE").pipe(gulp.dest(DIST_DIR)),
        gulp
          .src(GENERIC_DIR + "web/cmaps/**/*", { base: GENERIC_DIR + "web" })
          .pipe(gulp.dest(DIST_DIR)),
        gulp
          .src(GENERIC_DIR + "web/standard_fonts/**/*", {
            base: GENERIC_DIR + "web",
          })
          .pipe(gulp.dest(DIST_DIR)),