Add binding for Terminate method (#58)

Thank you for the contribution :)

Author: JustAman62

src/SharpWebview/Webview.cs

@@ -187,6 +187,14 @@ public IntPtr GetWindow()
             return Bindings.webview_get_window(_nativeWebview);
         }
 
+        /// <summary>
+        /// Terminates the webview.
+        /// </summary>
+        public void Terminate()
+        {
+            Bindings.webview_terminate(_nativeWebview);
+        }
+
         /// <summary>
         /// Disposes the current webview.
         /// </summary>