Add index to legendItem interface (#10436)

* Add index to legendItem interface for doughnut, pie and polarArea charts. Make datasetIndex optional since the before named charts dont include it.

* Remove test to check if datasetIndex has been set in generateLabels function for legend

types/index.esm.d.ts

@@ -2165,7 +2165,12 @@ export interface LegendItem {
   /**
    * Index of the associated dataset
    */
-  datasetIndex: number;
+  datasetIndex?: number;
+
+  /**
+   * Index the associated label in the labels array
+   */
+  index?: number
 
   /**
    * Fill style of the legend box

types/tests/plugins/defaults.ts

@@ -9,10 +9,3 @@ defaults.plugins.legend.labels.generateLabels = function(chart) {
     text: 'test'
   }];
 };
-
-// @ts-expect-error Type '{ text: string; }[]' is not assignable to type 'LegendItem[]'.
-defaults.plugins.legend.labels.generateLabels = function(chart) {
-  return [{
-    text: 'test'
-  }];
-};