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
gulp.task(
  "mozcentraldiff",
  gulp.series(
    "mozcentral",
    "mozcentralbaseline",
    function createMozcentralDiff(done) {
      console.log();
      console.log("### Creating mozcentral diff");
      
      
      
      
      
      rimraf.sync(MOZCENTRAL_BASELINE_DIR + "*");
      gulp
        .src([BUILD_DIR + "mozcentral/**/*"])
        .pipe(gulp.dest(MOZCENTRAL_BASELINE_DIR))
        .on("end", function () {
          safeSpawnSync("git", ["add", "-A"], { cwd: MOZCENTRAL_BASELINE_DIR });
          const diff = safeSpawnSync(
            "git",
            ["diff", "--binary", "--cached", "--unified=8"],
            { cwd: MOZCENTRAL_BASELINE_DIR }
          ).stdout;
          createStringSource(MOZCENTRAL_DIFF_FILE, diff)
            .pipe(gulp.dest(BUILD_DIR))
            .on("end", function () {
              console.log(
                "Result diff can be found at " +
                  BUILD_DIR +