[Search] a11y make index mappings search results accessible (elastic#…

…201455)

## Summary
Make index mappings search results accessible so that it can read by
screen reader when navigated by keyboard

x-pack/plugins/index_management/public/application/components/mappings_editor/components/document_fields/search_fields/search_result_item.tsx

@@ -108,6 +108,7 @@ export const SearchResultItem = React.memo(function FieldListItemFlatComponent({
               'mappingsEditor__fieldsListItem__content--toggle': hasChildFields || hasMultiFields,
               'mappingsEditor__fieldsListItem__content--multiField': isMultiField,
             })}
+            tabIndex={0}
           >
             <EuiFlexItem
               grow={false}