Add missing typescript property declaration

The method getEngineName exists both in the project and the official docs. It's just missing in the typescript declaration file.

index.d.ts

@@ -105,6 +105,13 @@ declare namespace Bowser {
 
       getEngine(): EngineDetails;
 
+      /**
+       * Get parsed engine's name
+       * @returns {String} Engine's name or an empty string
+       */
+
+      getEngineName(): string;
+
       /**
        * Get parsed result
        * @return {ParsedResult}