fix: make datepicker more accessible (#3343)

A finding of an a11y test on a project, where the Datepicker is used, was that the proposed columnheader role is missing, thus making the datepicker not accessible for blind users.

src/datepicker/datepicker-month-view.ts

@@ -12,7 +12,7 @@ import {DayTemplateContext} from './datepicker-day-template-context';
   template: `
     <div *ngIf="showWeekdays" class="ngb-dp-week ngb-dp-weekdays bg-light">
       <div *ngIf="showWeekNumbers" class="ngb-dp-weekday ngb-dp-showweek"></div>
-      <div *ngFor="let w of month.weekdays" class="ngb-dp-weekday small">
+      <div *ngFor="let w of month.weekdays" class="ngb-dp-weekday small" role="columnheader">
         {{ i18n.getWeekdayShortName(w) }}
       </div>
     </div>