[chart.js] the text property of title options can be string[] (#24923)

See the [chart.js
documentation](http://www.chartjs.org/docs/latest/configuration/title.html):
"if specified as an array, text is rendered on multiple lines."

types/chart.js/chart.js-tests.ts

@@ -24,6 +24,9 @@ const chart: Chart = new Chart(new CanvasRenderingContext2D(), {
         onHover(ev: MouseEvent, points: any[]) {
           return;
         },
+        title: {
+            text: ["foo", "bar"]
+        },
         tooltips: {
             filter: data => Number(data.yLabel) > 0,
             intersect: true,

types/chart.js/index.d.ts

@@ -219,7 +219,7 @@ declare namespace Chart {
         fontColor?: ChartColor;
         fontStyle?: string;
         padding?: number;
-        text?: string;
+        text?: string | string[];
     }
 
     interface ChartLegendOptions {