Webpack: Make Resolve.alias optional

The alias property on the Resolve interface was incorrectly declared
as required. This commit makes it optional.

Files changed:
 - webpack/webpack.d.ts: Set Resolve.alias to optional
 - webpack/webpack-tests.ts: Add new test which would have failed
   but now passes

Documentation: https://webpack.github.io/docs/configuration.html#resolve-alias

webpack/webpack-tests.ts

@@ -247,6 +247,12 @@ configuration = {
     devtool: "#inline-source-map"
 };
 
+configuration = {
+  resolve: {
+    root: __dirname
+  }
+};
+
 loader = {
     test: /\.jsx$/,
     include: [