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
return gulp.src(
    [
      "external/standard_fonts/*.pfb",
      "external/standard_fonts/*.ttf",
      "external/standard_fonts/LICENSE_FOXIT",
      "external/standard_fonts/LICENSE_LIBERATION",
    ],
    {
      base: "external/standard_fonts",
    }
  );
}
function checkFile(filePath) {
  try {
    const stat = fs.lstatSync(filePath);
    return stat.isFile();
  } catch (e) {
    return false;
  }
}
function checkDir(dirPath) {
  try {
    const stat = fs.lstatSync(dirPath);
    return stat.isDirectory();
  } catch (e) {
    return false;
  }
}
function replaceInFile(filePath, find, replacement) {