Update index.ts (#13539)

Adding this fild will solve the problem of module build failed: Error:  Property 'defaults' does not exist on type 'typeof Chart'. And You can use this Chart.defaults.global in ts

chart.js/index.d.ts

@@ -392,6 +392,7 @@ interface RadialLinearScale {
 }
 
 declare class Chart {
+  static defaults: any;
     constructor (context: CanvasRenderingContext2D | HTMLCanvasElement, options: ChartConfiguration);
     config: ChartConfiguration;
     data: ChartData;