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
MOZCENTRAL_DIFF_FILE
              );
              done();
            });
        });
    }
  )
);
gulp.task("externaltest", function (done) {
  console.log();
  console.log("### Running test-fixtures.js");
  safeSpawnSync("node", ["external/builder/test-fixtures.js"], {
    stdio: "inherit",
  });
  console.log();
  console.log("### Running test-fixtures_esprima.js");
  safeSpawnSync("node", ["external/builder/test-fixtures_esprima.js"], {
    stdio: "inherit",
  });
  done();
});
gulp.task(
  "npm-test",
  gulp.series(
    gulp.parallel("lint", "externaltest", "unittestcli"),
    "lint-chromium",
    "typestest"
  )
);