fix(custom): use function instead of class

Author: Ovilia

src/chart/custom/customSeriesRegister.ts

@@ -0,0 +1,11 @@
+import { CustomSeriesRenderItem } from './CustomSeries';
+
+const customRenderers: {[type: string]: CustomSeriesRenderItem} = {};
+
+export function registerCustomSeries(type: string, renderItem: CustomSeriesRenderItem): void {
+    customRenderers[type] = renderItem;
+}
+
+export function getCustomSeries(type: string): CustomSeriesRenderItem {
+    return customRenderers[type];
+}