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
* limitations under the License.
 */
"use strict";
const autoprefixer = require("autoprefixer");
const calc = require("postcss-calc");
const fs = require("fs");
const gulp = require("gulp");
const postcss = require("gulp-postcss");
const rename = require("gulp-rename");
const replace = require("gulp-replace");
const mkdirp = require("mkdirp");
const path = require("path");
const rimraf = require("rimraf");
const stream = require("stream");
const exec = require("child_process").exec;
const spawn = require("child_process").spawn;
const spawnSync = require("child_process").spawnSync;
const streamqueue = require("streamqueue");
const merge = require("merge-stream");
const zip = require("gulp-zip");
const webpack2 = require("webpack");
const webpackStream = require("webpack-stream");
const Vinyl = require("vinyl");
const vfs = require("vinyl-fs");
const through = require("through2");
const BUILD_DIR = "build/";
const L10N_DIR = "l10n/";
const TEST_DIR = "test/";
const EXTENSION_SRC_DIR = "extensions/";
const BASELINE_DIR = BUILD_DIR + "baseline/";