Update highcharts-ng loading property

Per Issue #8691, the loading property can be a boolean or a string value. Add the string type as a possibility for this property.

highcharts-ng/highcharts-ng.d.ts

@@ -17,7 +17,7 @@ interface HighChartsNGConfig {
     };
     //Boolean to control showng loading status on chart (optional)
     //Could be a string if you want to show specific loading text.
-    loading?: boolean;
+    loading?: boolean | string;
     //Configuration for the xAxis (optional). Currently only one x axis can be dynamically controlled.
     //properties currentMin and currentMax provied 2-way binding to the chart's maximimum and minimum
     xAxis?: {