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
const MOZCENTRAL_BASELINE_DIR = BUILD_DIR + "mozcentral.baseline/";
const GENERIC_DIR = BUILD_DIR + "generic/";
const GENERIC_LEGACY_DIR = BUILD_DIR + "generic-legacy/";
const COMPONENTS_DIR = BUILD_DIR + "components/";
const COMPONENTS_LEGACY_DIR = BUILD_DIR + "components-legacy/";
const IMAGE_DECODERS_DIR = BUILD_DIR + "image_decoders/";
const IMAGE_DECODERS_LEGACY_DIR = BUILD_DIR + "image_decoders-legacy/";
const DEFAULT_PREFERENCES_DIR = BUILD_DIR + "default_preferences/";
const MINIFIED_DIR = BUILD_DIR + "minified/";
const MINIFIED_LEGACY_DIR = BUILD_DIR + "minified-legacy/";
const JSDOC_BUILD_DIR = BUILD_DIR + "jsdoc/";
const GH_PAGES_DIR = BUILD_DIR + "gh-pages/";
const SRC_DIR = "src/";
const LIB_DIR = BUILD_DIR + "lib/";
const DIST_DIR = BUILD_DIR + "dist/";
const TYPES_DIR = BUILD_DIR + "types/";
const TMP_DIR = BUILD_DIR + "tmp/";
const TYPESTEST_DIR = BUILD_DIR + "typestest/";
const COMMON_WEB_FILES = ["web/images/*.{png,svg,gif,cur}", "web/debugger.js"];
const MOZCENTRAL_DIFF_FILE = "mozcentral.diff";
const REPO = "git@github.com:mozilla/pdf.js.git";
const DIST_REPO_URL = "https://github.com/mozilla/pdfjs-dist";
const builder = require("./external/builder/builder.js");
const CONFIG_FILE = "pdfjs.config";
const config = JSON.parse(fs.readFileSync(CONFIG_FILE).toString());
const AUTOPREFIXER_CONFIG = {
  overrideBrowserslist: [
    "last 2 versions",
    "Chrome >= 68",
    "Firefox ESR",