onMissingChildren

[ChristopherMeek]

Hey All,

I'm trying to put together a PR to complete the onMissingChildren functionality. Modifying TreeItem to do a check similar to the missing item check is straightforward enough, something like ...

if(item.hasChildren && isExpanded && item.children === undefined && !hasRequestedChildren) {
  setHasRequestedChildren(true);
  environment.onMissingChildren?.([props.itemIndex]);
}

This works fine for controlled environments. For uncontrolled environments a bit more work needs to be done to the dataprovider and UncontrolledEnvironment. I'm thinking of adding getChildren?: (itemId: TreeItemIndex) => Promise<TreeItemIndex[]>; to the TreeDataProvider interface and then an onMissingChildren handler to UncontrolledEnvironment that pushes the received ids into item in much the same way as onMissingItems.

Of course I will add tests, test-data and stories to complete the PR.

Does this sound like a reasonable approach?