Add client side scripting support

[iisisrael]

• Add TypeScript configuration to webpack.config.js
  • Not sure how this should be implemented using Encore, but a standard webpack config might look something like:

module.exports = {
    module: {
        rules: [
            {
                test: /\.ts$/,
                loader: 'ts-loader'
            }
        ]
    }
}

• Add Mocha test suite
  • Add jsdom, jsdom-global, mocha, and sinon as dev dependencies in package.json
  • Add test build and run scripts in package.json - again, not sure how this should work with Encore, but with a standard webpack implementation this might look something like:

"scripts": {
    "buildtest": "./node_modules/webpack/bin/webpack.js --progress --config webpack.test.config.js",
    "test": "npm run buildtest && mocha ./testjs/tests"
}

• See this example test to get started