Allow getTreeItem be Thenable

[rebornix]

re #27467

getChildren is Thenable but getTreeItem is not. Sometimes we only do real business only when users click on TreeItem but right now we have to populate all data to TreeItem synchronously in getTreeItem or do that asynchronously in getChildren, none of them are perfect.

Take Git History view as example, getChildren always returns commit list or changed files list, which can be done in getChildren asynchronously. But when users click on a changed file, I want to save both the original file content and the modified content to local disk and feed them to vscode.diff to do diff, I don't have an async way to fetch/save those content.

getTreeItem(element: T): TreeItem | Thenable<TreeItem>;

will be really awesome.