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
preprocessHTML("web/viewer.html", defines).pipe(
          gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")
        ),
        preprocessCSS("web/viewer.css", "chrome", defines, true)
          .pipe(
            postcss([autoprefixer({ overrideBrowserslist: ["chrome >= 68"] })])
          )
          .pipe(gulp.dest(CHROME_BUILD_CONTENT_DIR + "web")),
        gulp.src("LICENSE").pipe(gulp.dest(CHROME_BUILD_DIR)),
        gulp
          .src("extensions/chromium/manifest.json")
          .pipe(replace(/\bPDFJSSCRIPT_VERSION\b/g, version))
          .pipe(gulp.dest(CHROME_BUILD_DIR)),
        gulp
          .src(
            [
              "extensions/chromium/**/*.{html,js,css,png}",
              "extensions/chromium/preferences_schema.json",
            ],
            { base: "extensions/chromium/" }
          )
          .pipe(gulp.dest(CHROME_BUILD_DIR)),
      ]);
    }
  )
);
gulp.task("jsdoc", function (done) {
  console.log();
  console.log("### Generating documentation (JSDoc)");
  const JSDOC_FILES = ["src/doc_helper.js", "src/display/api.js"];