Read the controllers.json as string rather than Buffer

Buffers can implicitly be converted to string (implicitly using `utf8`
as encoding) but the type checker does not like this kind of implicit
conversion.
When passing an encoding to `fs.readFileSync`, it returns a string by
performing an explicit conversion using the provided encoding.

lib/WebpackConfig.js

@@ -755,7 +755,7 @@ class WebpackConfig {
         }
 
         // Add configured entrypoints
-        const controllersData = JSON.parse(fs.readFileSync(controllerJsonPath));
+        const controllersData = JSON.parse(fs.readFileSync(controllerJsonPath, 'utf8'));
         const rootDir = path.dirname(path.resolve(controllerJsonPath));
 
         for (let name in controllersData.entrypoints) {