Webpack-cli required

[tguvdamm]

When building the exact example from the homepage in a yarn workspace with node 12 or 14, webpack warned me that webpack-cli was required to build the project.
I originally used the gas plugin with webpack in a larger project that probably somehow suppressed that warning and just resulted in an error similar to the one mentioned in #460 .

Proposal: add documentation to add webpack-cli to dependencies when using yarn.

---

tree:

.
├── echo.js
├── main.js
├── package.json
└── webpack.config.js

Package.json contents:

{
  "name": "temp",
  "version": "1.0.0",
  "description": "temp",
  "license": "MIT",
  "devDependencies": {
    "gas-webpack-plugin": "^2.0.0",
    "webpack": "^5.28.0"
  },
  "scripts": {
    "build": "yarn webpack"
  }
}

Other files are copied verbatim from this package's homepage example.