Initial TypeScript infrastructure (#2206)

This allows us to get started with converting all Flarum JavaScript code to TypeScript.
In addition, we will have time to experiment to find the best Webpack configuration before integrating into flarum-webpack-config.

See flarum/flarum-webpack-config#3.

js/webpack.config.js

@@ -5,4 +5,12 @@ module.exports = merge(config(), {
   output: {
     library: 'flarum.core'
   },
+
+  // temporary TS configuration
+  resolve: {
+    extensions: ['.ts', '.tsx', '.js', '.json'],
+  },
 });
+
+module.exports['module'].rules[0].test = /\.(tsx?|js)$/;
+module.exports['module'].rules[0].use.options.presets.push('@babel/preset-typescript');