doc(configuration): Extend TypeScript (#2628)

- Add link to DefinitelyTyped
- Add @types/webpack-dev-server

Author: simon04

src/content/configuration/configuration-languages.md

@@ -16,10 +16,12 @@ webpack accepts configuration files written in multiple programming and data lan
 
 ## TypeScript
 
-To write the webpack configuration in [TypeScript](http://www.typescriptlang.org/), you would first install the necessary dependencies:
+To write the webpack configuration in [TypeScript](http://www.typescriptlang.org/), you would first install the necessary dependencies, i.e., TypeScript and the relevant type definitions from the [DefinitelyTyped](https://definitelytyped.org/) project:
 
 ``` bash
 npm install --save-dev typescript ts-node @types/node @types/webpack
+# and, if using webpack-dev-server
+npm install --save-dev @types/webpack-dev-server
 ```
 
 and then proceed to write your configuration: