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
createCMapBundle().pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "web/cmaps")
        ),
        createStandardFontBundle().pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "web/standard_fonts")
        ),
        preprocessHTML("web/viewer.html", defines).pipe(
          gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")
        ),
        preprocessCSS("web/viewer.css", "mozcentral", defines, true)
          .pipe(
            postcss([
              autoprefixer({
                overrideBrowserslist: ["last 1 firefox versions"],
              }),
            ])
          )
          .pipe(gulp.dest(MOZCENTRAL_CONTENT_DIR + "web")),
        gulp
          .src("l10n/en-US/*.properties")
          .pipe(gulp.dest(MOZCENTRAL_L10N_DIR)),
        gulp
          .src(FIREFOX_EXTENSION_DIR + "README.mozilla")
          .pipe(replace(/\bPDFJSSCRIPT_VERSION\b/g, version))
          .pipe(replace(/\bPDFJSSCRIPT_COMMIT\b/g, commit))
          .pipe(gulp.dest(MOZCENTRAL_EXTENSION_DIR)),
        gulp.src("LICENSE").pipe(gulp.dest(MOZCENTRAL_EXTENSION_DIR)),
        gulp
          .src(FIREFOX_CONTENT_DIR + "PdfJsDefaultPreferences.jsm")
          .pipe(transform("utf8", preprocessDefaultPreferences))