fix(components/modals): set modal content tabindex to make scrollable area focusable (#619)

Author: Blackbaud-ErikaMcVey

libs/components/modals/src/lib/modules/modal/modal.component.html

@@ -59,7 +59,7 @@
     <div
       class="sky-modal-content sky-padding-even-large"
       role="region"
-      tabindex="-1"
+      tabindex="0"
       [attr.aria-labelledby]="modalHeaderId"
       [attr.id]="modalContentId"
       (skyModalScrollShadow)="scrollShadowChange($event)"